They are totally different guns, although they do have several similarities in design. The Quest is a copy of the Gamo series... there are a few dimensional differences, but may parts interchange.
Compared to the Quest, the BL1000 has a heavier spring, a larger powerplant, and a very heavy tophat - it definitely kicks harder, although the performance in .177 is comparable. Believe it or not, the metalwork on the Quest is actually superior. The BL also has a stiffer trigger.
As for those rear sights - the ones on the BL are no better than the Quest! As a matter of fact, they were positively driving me nuts on mine - the gun refused to shoot with any real consistency until I plopped a scope on it.
In my experience, the Quest seems less scope-friendly (same with my old B19). The barrel lock-up seems less consistent in the vertical direction... although that doesn't seem to impact shooting with the open sights. I suspect it'll get better as the breach seal breaks in. And yes, the standard Crosman scope mounts are inadequate on either rifle - although the come on both. It is possible to install a roll pin into the rear mount to engage the recoil anchor hole in the Benji's compression tube, that'll keep the rear mount from sliding back on the rail. The Quest actually has a scope stop, so the pin shouldn't be necessary. But the mounts really should be better for either of these rifles.
As for alternatives, the Daisy Powerline is a heckuva performer for the money - but it is a harsh and crude gun. And the scope comes with the same mounts as the Crosman! Down side to the Daisy - no parts support. Ya bust a mainspring, and you either send it back or fish around for something else.
Gamo's are good for the money - especially if you can get one of the refurb deals at Natchez or - better yet - a closeout deal on a new Shadow at Walmart. I got one for $99, another guy got one for $59! If you have a Walmart anywhere near you (and you don't live in NJ), it's worth stopping by.
Gamo's are made better than the Quest, the Legacy, or the Powerline. Unfortunately, Gamo parts support ain't much better than Daisy's - BUT the "consumables" (mainsprings and piston seals) can be replaced with Crosman Quest parts! And Gamo's do scope very well, assuming you get decent mounts.
As another poster suggested, the RWS34 seems to be a perrenial favorite - never had one, but I've heard lotsa good things about them...