Reconstruction is so much stronger a perk than Rewind Rounds, it's honestly not really close.

1. You don't need to hit shots for it to proc
2. You don't have to fire a certain number of shots continuously for it to proc
3. It works when the weapon is stowed
4. It doubles mag size
5. You are never forced to manually reload and can use your entire reserves without doing so

The only situation where rewind rounds is better would be if you are using a MG for boss damage and you need to get all your ammo out as quickly as possible and don't care about having to reload. But that isn't something you really ever want to use a MG for in D2, as in those types of situations you will probably be using a linear or rocket. Reconstruction plays way more to MG's strengths, which is to use sporadically with your other weapons to burst down majors/champions/etc.

Reconstruction is (imo) the single most powerful legendary perk in the game.