WarpSDL is no more "incomplete" than Novacoders new AGA SDL, in fact if anything Id have to say the contrary.
The "problem" with WarpSDL however is that the "dev kit" was never publicly released, ergo of no use to anyone.
Chaozer actually sent me the lib and headers some years back when I was chatting to him on IRC, but unfortunately I didnt really know at the time what I was doing, nor did I realise how valueable said files would be to me so I dont think I ever backed them up (very slim chance that I still have them, but even if thats true they could be anywhere).
Thanks to Novacoder though we now have a basic SDL lib for AGA, which has been fun to play with. Dont know if anyone is interested, but I've compiled Reminescence for 68k (flashback replacement engine), along with Bermuda Syndrome (and a few others I abandonded for lack of performance reasons).
As for the original question it depends on what you want to do and what type of languages a person is familiar with/wants to learn. The scope of developing for amiga/amiga based systems is way too broad to give a simple answer. Some sort of elaboration might yield responses closer to what the OP was specifically hoping for.