Also note that next year, there IS a similar app coming out called Construct ( http://www.scirra.com ), which is being designed by Tigerworks, Herb and a few other ex-extension developers.
So you may choose to wait and use 1.5 for a little longer, or get MMF2 now and maybe get Construct once it's had a year to mature. Construct's Lite version may be as cheap as £45 though.
It's still in development, but a public beta's coming out in the Summer (roughly September time I think they suggested).
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.