I have to agree with Cheese, I have the ETS EC 150/3 and I absolutely love that sander. I've used it with everything from 40 grit through 600 grit abrasives and it is an absolute pleasure to use. I've used it for hours on end and it causes the least amount of fatigue of any sander I've ever used. To me, this is the ultimate all around sander. However, I typically build furniture so I'm never stripping paint or sanding very rough lumber - I would imagine if you're typically doing that kind of heavy sanding you might want the ETS EC 150/5.
I ordered a Rotex 150, but after a weekend of working on several large walnut slabs, I returned it because I found it a very heavy and unruly sander requiring two hands at all times. Very tiring to use. It also has a tendency to jump around on the work piece. I really wanted to add the Rotex to compliment my ETS EC 150/3, but, after that initial disappointing experience, I didn't want to ever use it again. I much prefer the ETC EC even for heavy sanding.
I have to admit that I did order the Pro 5 as an impulse buy. I honestly have no need for it, I just got caught up in the excitement of the promotion. I know that sounds pretty stupid of me, but that's the honest truth. I do intend to use the voucher to purchase a DTS 400 to get into the corners my ETS EC 150/3 can't reach, plus I'm looking forward to getting the blue and green systainer. I'm hoping I might see some difference in the finish between the 2mm Pro 5 and the 3 mm ETS EC 150/3, but I doubt I'll really notice any. I realize this is a pretty goofy reason to buy the Pro 5 - guess I'm guilty as charged.
I hope some of this is helpful,
Steve