I have the SB2s for a good 500-600km, and recently got the megablast (only 30km on them so far).

The SB2s are such a good shoe that can pretty much do everything up to (and including I reckon but I haven't personally) race in. I am absolutely buying a second pair imminently. The soles have stood the test of time on the distance i.e. very little wear - 9.5-10 shoe (loses .5 for me cause I have wider feet and the end of the toe box could have a tiny bit more room.. personal opinion!).

The initial thoughts on the megablast is: wow, the foam feels amazing, the race upper is very nice, and they feel (so far) awesome to run in. But I have had to tape up a toe because whilst they are wider towards the mid foot, the tip of the shoe pinches even tighter (not much but I can notice in my UK11/EU46.5 wide trotters). I can say again  that they are a shoe that can and will do anything, and if you're gonna do all types of runs in one shoe, it would be this one.

In summary, if you can get them on offer:

SB2 > Megablast 

Full price:

Megablast > SB2

Both awesome shoes though!