Go as deep into music theory as you want, but it's not a numbers game rather a sound game, and Megadeth's songs, good as some may be, have by no means better songwriting than COB's. It's not as extreme as MOP's, but their structure and riff usage is very simple very often, and very few albums have no fillers, whereas for Bodom no album really has (Lobodomy should've probably never made it to the album, being that they themselves disliked it so much they "made a pact to never ever play it live because it was weird", and maybe the case can be made too for DWEDFN, but other than that...). Even AYDY which has been bashed to death has pretty darn good songs: IYWPPFW, Next In Line and Bastards Of Bodom are great songs, and to this day still puzzles me why don't they play them (outside IYWPPFW because maybe it's too much for Alexi to do both simultaneously, but then again the play SKO).
As far as the solos go, they may, at least some of them, not be as technical anymore, but tbh I couldn't care less as long as they fit the song and sound good. E.g., ADK's pre-solo, the slow one Alexi and Janne play together right before the actual solo, is an outstanding solo, because it fits perfectly, it has the almighty "feelign", it suits extremely well the song, and it's overall great, yet technicality is nowhere to be found. Same goes to ETID's solo or IYF's solo: technicality nowhere to be found really, yet I wouldn't want anybody else to go all technical on it because I'm pretty sure it wouldn't turn out better. With that I mean to say: Alexi's chops are right up there with Friedman's (maybe not with Broderick's, but he spent too much time locked up either in the gym or in a basement with Jeff Loomis guitar-fapping while the rest of the cool kids partied and so on), and he's proved it enough (first three albums plus some other solos, and 3 outstanding Sinergy albums), so hats of for trying something new and not sticking over and over again with the 5-6 strings e-minor sweeps and blues-pentatonic variations and getting outside of his "let's play super fast solos "comfort zone" (don't know if you've ever checked it out, but he did a guest solo on Pain's Just Think Again, extremely slow, but incredibly cool). We could argue over and over, but besides their technical levels being on par, determining which solos are better is sort of not so objective at this point. It's not as if we were e.g. comparing Kerry King's solos with Steve Vai's.