I have both.
A $200 Pro5 (about what the regular price will be) is not the same as the $400 ec125.
I love having the ec125/3. Bought it last summer.
I set the 2mm swirl unit with the soft pad, the 3mm swirl unit with the hard pad. Nice combination so far.
Had I known I was getting the pro5, I would have bought the 150/5 as a better compliment.
Now that I've had time to fondle the x400 line, the triangle model might be a better general purpose sander.
You're luckier than you realize. The ec 150 pad will mount to the ec 125, but not vice versa. And, yes, the ec 150 + pro5 is a nice combination, indeed.
Heck, an ec 125/150 + pro5 should be sweeeeet.
Concerning 3mm vs 5mm... I wouldn't worry about it.
I'm sure you've noticed the power that the EC delivers.