There is no best choice... you can't go wrong with either one.
I think if you just had a 35mm and a 21mm you might find the gap too large... but it depends what you like to shoot.
Using a 21mm for people allows you to get really close to the subject which is great if you like that look, but if you don't feel comfortable getting that close than I think the 25mm is a better choice.
Of course if you have just the 35 and 21 then in two lenses and a lightweight kit you have covered a large range of lenses.
-Rob
Love the hammock shot. We're just about to send a 21/4.5 back to Zeiss. Lovely lens. If I didn't already have two 21mm lenses for the Leicas I'd seriously consider it.
Then again, if I could afford the 16-18-21 Tri-Elmar there'd be no contest. The Biogon has far less distortion, but the Tri-Elmar distortion is still tiny and it's a gorgeous lens.