You are thinking of an SD-card like a film canister, where a film would hold 24 or 36 pictures give or take how careful you were with the lead-in ;-) Trouble is in digital, camera doesn't 'take' a picture and record an 'image'.. it takes gazzillions of light-meter 'spot' readings (the brightness of each 'pixel'), and records the 'number' for each, so that what-ever you want to look at your picture on can re-create it by 'painting by numbers'... so you don't have any 'pictures' on an SD card... you have a file containing the 'paint by number' instructions. Each 'digit' used to record a brightness value, takes up a 'byte' of space on your memory card. You need three brightness values, one for each colour, red, blue & Green, to define a pixel, so more pixels in your 'image' so the more bytes of memory a picture will take up; but, you also need a few bytes of data to tell the viewing widget where each pixel belongs, and the camera can use 'compression'.. computer short-hand for want of a better description, to make file-sizes smaller;
so there is no direct correlation between Maga-Pixels and Mega-Bytes.. and when the camera tells you how many shots you have left on a card, if you look in the manual, it will tell you its only an 'estimate'.
My camera has 24Mega-Pixels; makes J-Pegs about 10Mega-Bytes or NEF's about 100Mega-Bytes.. but look at a 'set' in file manager on 'Details' rather than as icons, and look at the file-size column, they are rarely exactly the same size, and can vary in file-size by maybe as much as 30% or more 'as shot', purely from how much data the camera has had to record to get all the detail in the picture I took, or not! - More elaborate explanation, if you are interested, here:-
Understanding; Mega Pixels, Mega Bytes; tif, jpg, bmp, nef, & raw?
As for the card access times; this is also not related to the cards 'size'; The card is like a car-park; the size in GB tells you how many cars you might park in it... but remember, you are parking paint by numbers instructions not pictures, so you might squeeze a couple of Smart-Cars in one bay or have to park a new Mini across two of them
... Having LOST my car in the NAI multi-story when it first opened umpety decades ago, would stand to reason that the bigger the card, the more 'hunting' whatever is reading or writing to it will have to do to find a parking space, so you would expect a bigger card to be 'slower' to get stuff in or out... BUT, if you have good bay marking (unlike the NIA!) could find your space or car dead easy; then you just have to get in and out, and again, if you have a five lane entrance; could be pretty swift, compared to having a single turn pike for in and out! This is where the card 'Class' comes in; there's a few different ways they mark card-speed; and some of the faster cards actually state the read/write speed; My Scan-Disk Extremes say 45MB/S.. ie 45 Mega-Bites per second; but its also marked with a 10 in a circle and a 1 in a bucket. For note, the bigger the number in a circle, the faster the card (I think a circle 10 is up to about 30MB/s) the buckets go the other way, and a 1 in a bucket is slower than a 3 in a bucket; but the 'better' cards this quick will probably be marked with actual 'max' data rate. These different class symbols are because of different 'standards' for SD-Cards that don't just define the card 'speed' but also the reliability and other 'stuff' that's probably not so important to the question you have, just yet.
But brings us to Micro-SD's; and generally they will be more expensive for the same size and class; but it's in the 'other-stuff' the class defines that they start to fall down. Being small they have less surface area to dissipate heat; so even where they may be as fast as a full size SD card, they tend to be less reliable or long lasting. Full size SD card in my camera, under 'heavy' use can get very bleedin hot! a Micro-SD sleeved in an adapter? Going to suffer even more. Personally I wouldn't use them, and experience of them is that they aren't 'as fast' as full size SD cards of the same speed rating; it's a bit like cars 'power' rating; I used to have a 2.25l Land Rover with 60bhp, and a 1.3 Metro with a similar 60bhp... try driving either of them up a hill fully loaded! The Land-Rover had 60 Cart-Horse power, the Metro 60 Paleminos! Putting a large computer game the missus wanted to move to a different PC on her Micro-SD key-ring thing, took three times as long as sticking it on the same size & class full-size SD card out of my camera; simply because the smaller card couldn't sustain it's 'peak' transfer rate so long.
So conclusion to your conundrum, as already suggested, is basically full-size Scan-disk Extreme's with the higher transfer rates, and the 16GB 45/MB/s are a bit of an optimum for useful capacity, speed and price at the moment (about £10 or so per card), and unless your camera can write any faster than the card, and you are buffering out regularly when machine gunning.
But worth noting if you are duel writing in different formats; the NEFS are approx 10x the file size of the J-Pegs, so will take 10x as long to write, and you'll get 1/10th the shots per card; so you may consider an 'asymetric' set up with a smaller, slower card in the J-Peg slot... or why you are shooting in duel format?.... writing one file, in one format is always going to be faster and smaller than two.. and I believe you can 'schedule' second card 'back-up' on switch off on many duel card cameras if it is just for 'back-up'; and if you are shooting NEF to post-process, then why do you want a J-Peg out the camera? surely you can make a J-Peg copy in PP when you clear the card down, if you want that second format set? But that's taking matters off on another topic, and I mainly shoot J-Peg! Cos I don't see much point spending umpety hundred quid on a really sophisticated 'do it all for you' camera, to shoot 'raw' in 'manual' cos it's the 'done thing' making all its electrickery rather redundant, and give myself as much work to do as shooting my forty year old meter-less Zenit film camera!... actually more.. 'cos I'd only get 36 pictures out the Zenit, not 3600 to have to process! ;-) But that is an approach 'thing' and me, as an engineer, one of my early design lessons resonates still; engineers are applied technologists; we use science to solve problems and make life easy... when it's making MORE problems, and making life HARDER... its NOT 'technology'.. its a pain in the proverbial! And to me 'digital' photography is all about convenience, and making work for myself, shooting pictures I
have to post-process before I can do anything with, isn't really.. so I don't! But, your approach may be different; but still merit in KIS thinking... Keep-It-Simple if you can.
