I think that the answer depends on:
1) Live or home use? If mainly live then 98.34% or your audience won't know or care about how you get your tone. Get the axe-fx. Its easier to lug around as long as you are able to plug direct into FOH.
2) If mainly home use, are you quite fussy with your tone? If so then I think that the M4 rig will deliver better than the axe-fx with the right modules and poweramp.
I've played on the axe-fx and I own the Egnater setup with 12 or so modded modules. I prefer my modular setup by fair bit. Better feel, juicier, better harmonics... The modular set is extremely capable at low volumes as well. The downside for some people is that if you are a perfectionist you can spend a lot of time and money buying/selling modules modded by Jaded Faith, Gigamods, Salvation etc... The rewards are big though when you find the modules that really do it for you!
Both rigs are awesome when put together properly. Ultimately it depends on 1) and 2) above and how YOU hear things.