This is based on my own personal preferences but I think it would suit your requirements nicely...
The A1200 definitely, faster, easy enough to run both ECS & AGA games on (especially with WHDLoad) easier to expand memory, accelerator card, HD, USB etc...
As for coding I'd go for 68K assembler, fairly easy to learn and you'll get the best speed possible out of anything you may write...
Floppies (very rarely use them) but I'm pretty sure as far as I can remember that 1.44MB ones work fine...
