i personally have done both methods several times since this thread was started and found both methods to be pretty easy. the heat gun was more convenient tho imo.
the oven is nice and easy like everyone said heats the whole thing up evenly, but if over done it'll warp the **** out of your headlights (you gotta be a totally idiot to do that tho)
the heat gun to me is a faster process, heats up the headlight edges which is the only part that needs to be heated up. the faster it gets heated up the faster it comes apart...
in the end i guess its all just preference, either way is legit.