Especially once you get the latest builds and patches, 2 has some nice advances on 1.5.
Immediate conditions are coloured to distinguish them from standard conditions. So you know which events are going to be run the moment they're triggered (like clicking a button, or the triggers to fastloops) and which ones wait to be cycled in the event list (Always, object overlaps, etc).
And the latest build has OR conditions as well, so you can perform some more event logic. Hopefully future builds will feature stuff like while and else too.
Just be warned that you aren't likely to get an unbiased GM vs. MMF opinion here.
That said, if you already have MMF1.5 there's not really much reason to pay for MMF2 unless there's a particular feature you need. MMF2 does have a few interface improvements (and a very few functional improvements, such as scaling/rotation and layers), but there's not much you can do in MMF2 that you can't in MMF1.5.
I should mention that I have fuck all experience with Gamemaker, but I know it is fairly popular, so my (uneducated) recommendation is that if you're going to spend the money on one or another and you already have MMF you might as well get GM7, especially due to the price and also the simple fact that you'll have more options if you have both products.