Hi David,
I have the Peli 1560 wheeled version plus the 1520 and 1500 for different events. Can't comment on the 1510 really but obvious falls between the 1500-1520.
The 1560 will swallow everything without a problem (Nikon D3 & D3X Pro bodies, (and a Bronica on occasion), 70 - 200 f2.8 plus four other reasonable sized pro lenses). Whilst the 1560 accommodates everything, its really not necessary for most jobs, too heavy to drag around and do I really need everything else?
No.
Its a good 'go to', when I have that other accessory; the car. It would also make a good podium if you need to stand up but other than that, it is a bit large for general walking around.
The 1520 is a much better all rounder, whilst the 1500 mainly is for my speed lights etc but on occasion a bit small and often the backpack replaces it. You probably can get all you need in the smaller case but I am not au fait with the Canon 300mm in terms of size.
Also consider whether you might ever need to fly with them at some stage in the future.
Whilst I have talked my way onto a flight with the 1560 as hand baggage, this would probably be a no no nowadays and the smaller cases are more feasible although it all depends on the airline in question.