I already have Heroes 3(came with LE of Heroes 5) so I was wondering if the two expansions are worth $10 or if I should just stick with what I have.
Also how good is Heroes 2 compared to 3?

In terms of mechanics, the main thing Armageddon's Blade added was a new faction (the Conflux), along with a small assortment of new artifacts and creatures. The main thing Shadow of Death added was combination artifacts, again with a small assortment of new artifacts, creatures, and terrain types. Each expansion also came with a new campaign about equal in length to the original campaign. I'd personally consider the largest benefit being able to play some of the player-made maps requiring the expansions (and there are quite a few very good ones). It's also a good way to breath some new life into the game if you enjoyed it but feel that you've just about played it to death. If I had to give a shorter answer: yes, it's worth the $10.
As for HOMM2 compared to HOMM3, the mechanics are quite a bit simpler, but the game has a definite charm to it that's tough to put into words. Overall I like HOMM3 more, but still would highly recommend HOMM2.

If you finished the version that came with HoMM5 and want more, it'll be worth picking up the complete edition. There are dozens of extra hours of entertainment to be had.
The new features don't really make a huge difference to the game, so if you weren't satisfied by the copy you have, you probably won't get much more out of the complete edition.
HoMM2 had a bit more of a cohesive storyline, and two very obvious "good" and "evil" campaigns which give the game a bit more atmosphere (a bit more like HoMM5, in that sense). As with playing any other prequel, you'll really miss some of the features that were added in HoMM3.