For the last year or so, I chose pieces on this basis: Listen -> Like -> Get sheet music -> see whether it's at least slightly manageable (so far, always turned out to be...) -> Explore the piece, w/score only, then at the piano -> See whether it's reasonably manageable (so far, always...) -> Dig out the Bernhard posts which deal with practicing (Thanks to m1469's most excellent index, this doesn't take long anymore...

) and follow them up to the point where I need my teacher's advice (usually when I know the piece fairly well, memorized and at slow tempo HT. That's, however, not always the stage, sometimes it's earlier, somtimes later on...) -> Continue working on it with the teacher -> Finish it. (That's the best stage...

)
That works perfectly for me... I think one of the main things that makes me practice is the knowledge "I'm playing what I really like." So far, with this approach, I ran into no difficulties with my teacher... I have complete freedom in choosing my repertoire, and she doesn't have to be digging through libraries for repertoire suggestions...

So, to answer the original question from my point of view: It is better to do pieces you'd be willing to spend countless hours on, as long as those countless hours yield visible (or, better, audible...

) results. It's probably pointless to learn the Liszt sonata like that, but, at the same time, if you really like op. 90, there's no reason why not to work on it. I know very, very few feelings that equal mastering and performing the pieces I really like and I've spent hours and hours on. It's one hell of motivation to learn the next one.

Compared with "Whew, that hated [insert your own forced-to-practice piece here] is finally behind me", I think it's clear... The only limitations are time and how well you can estimate your abilities (The latter usually being a matter of one or two hours for the particular piece, sometimes, it's just obvious from the score...).
Just my $0.02.
You could always learn the Beethoven sonatas you like (under your own steam) alongside your main pieces (without telling the teacher - he! he!).
That's how I'm learning the TEs. Sssh!
