I have a PSP SLIM and a PSP Go! myself.

I would say to keep them both, if you can afford to.
Half Byte Loader r85 has some great homebrew compatibility.  

However, PSP SLIM beats it hands down in all areas.
Including design IMO.
The little Go! does indeed feel a little too cramped for my big hands.

So if you have the loot, i say keep them both.
If you choose between the two, then pick the SLIM  
Of course there is always another option:
Get the SLIM, sell the Go!, and use the proceeds from the Go! to buy a CR5400 adapter and two 16GB SDHC cards.    

32 GB on a PSP SLIM for the win