Thanks for the input everyone. For no reason that I can out my thumb on I was sort of hoping the DS was good enough for what I was wanting. Ive never really been a big fan of Nintendo machines apart from the GBA, but prior to asking questions I was leaning towards the DS.
I must admit to being a bit surprised that the DS is so underwhelming when it comes to emulation. Not that I was expecting either to run latest mame, or saturn, dreamcast, etc., but Im surprised that even the 16bit consoles were out of reach for the DS.
While there's a smaller library of PSP vs. DS games for the most part it appears its (PSP) native games are of more interest to me. I quite like the Wipeout and Gods Of War games.
The thing that I like about the NDS though is that it appears to be the last system around that has a good choice of retro/2d style games (but a little enhanced). Being that I'd mostly use it to amuse myself whilst on long train/bus rides that was a big of a consideration (and why emulation is a big part in my choice).
So, all said and done, I suspect I'll go the PSP route. Im far from the most decisive person around though, so who knows, that may change when I actually make the purchase
Thanks again everyone.