I have substantial experience with Oade Warm- and Transparent-modded UA5s. But I don't have any direct FR2LE experience...yet. However, most of the recordings I've heard sound mighty fine, and if they haven't I've chalked it up to cruddy sounding room, mic placement, etc., not the recorder itself. Nonetheless, I thought I'd chime in.
I think unless one intends to record unamplified sources requiring very high, very clean gain (in which case I might not go with either of the two options), it won't make a huge difference whether you run UA5 > recorder or FR2LE. The mics will play a far larger role in the overall sound. I really, really love the Oade Warm- and Transparent-PLUS modded UA5s. Simply stunning sound for the price, I think. Though, for my money, a little extra cash for the FR2LE would still sound good and make life easier: less stuff to carry around, fewer batteries to charge, fewer interconnects to sweat, more leeway in setting levels conservatively since running "24-bit", etc.