Hi Eddie,
Would that be JB, MC or little Pete who took the top shot? I reckon JB.....
In terms of lenses, I know a lot of carpers are switching over to lenses like the Sigma 30mm f/1.4 and whatever 50mm lenses are available for Canon. I had a play with Steve Renyard's 30mm Sigma the other week at Hunt's and was seriously impressed by the IQ. On Canon 1.6x crop it's not a bad focal length, although once you start to go longer (say 50mm) then you do start to limit your options because you tend to get one type of shot.
The top shot was done with a 17-40, as that's the standard lens on our kits, and I can see it's been done with the idea of getting cover straps but if it was just a record shot then you needn't be going as wide-angle. Exposure-wise it's underexposed but providing the ISO is low enough then it can be tweaked with some fill adjustment and contrast tweaks to make it 'pop' without incurring serious noise issues.
I personally like wide-angle shots for these big fish when shooting in portrait format, as you get a bit more of a dynamic shot and by the nature of wide-angle lenses giving you more perceived DoF, you don't have to be up at f/11 or similar.
The main issue aside from overall exposure, is where to focus. With mirrors it's easy, as you have sclaes to latch onto for contrast detection, but with leathers and mirrors it's harder to get the camera to pick up a specific focus point, especially in dull conditions. The good thing about big fish like the one you're holding is you can't hold them far from your body so the plane of focus can pretty much run through your face down to the fish. I usually take a few options; one focussed on the angler's face to capitalise on this, then one focussed on the flank of the fish.
You do need to be aware of just how angled the fish is to the camera; if it's a head-on shot then it's a lot harder, as you end up requiring huge DoF and in crappy light that is hard (unless you have mega high ISOs to play with). Try to have the fish as flat to the camera as possible to give yourself the best chance of everything being in focus and sharp.
The second shot is the standard record shot that we'd use mid-feature and is the one most anglers take as a memory of the fish.
Personally, I prefer the angler to have one knee on the ground, as this allows them to tilt the fish (drop the tail, raise the head or whatever) and it doesn't make them look like they're squatting to have a dump

This also means they're not having to keep they're balance, which means they don't have to hold the fish out as much. Keeping the fish closer to the body means you can use narrower DoF (handy when you have a busy background) but it has to be a comfortable, natural-looking pose.
Lens-wise for this shot I would be tempted to stay away from anything wider than 20mm and anything longer than 80mm, unless you're totally confident that the photographer (if you are the fish holder) will nail the DoF to get both the angler and fish sharp. Wide lenses just make the fish look bizarre, especially if it's a deep fish. I do use a 17mm lens for this kind of shot when we just want to get the head of the fish in focus for purposes such as showing the hook bait still in the mouth. But for record shots where the angler plays as much importance as the fish, I'd keep the fish flat to the camera and use about 30mm.
I have done shots of fish being held like this on a 200mm lens and they look the nuts when there's a clean background but that's when I have the luxury of space, and plenty of light to get a fast enough shutter speed AND get a decent aperture of around f/5.6. Then I ask the angler to keep the fish tight to him and focus somewhere near the dorsal - that's should get everything sharp.
As for your lens dilemma, If I were to go for either the 50mm or the 18-55, I'd go with the zoom every day, just for the versatility. If it's your only lens then having just a prime can reduce your options. They may be good in low light but these days, with cameras like the 60D that can comfortably do ISO 6400 (and it be printable at A3) there isn't that much of an argument for using f/1.8 glass.
Hope this helps and don't hesitate to ask me anything - if I can help I will