Need more information on what you're cutting. I've handled an MS170, but that's about it. Picking it up and looking it over was enough to make me put it back down. It's certainly lighter than an MS261, but it's also a lot slower which more than offsets the weight difference for me.
I've never used a top handle saw (MS211) but the numbers say its barely lighter than the 261.
All that to say that I agree with the others. Work towards getting a 261 if you're wanting something lighter. Its substantially lighter than your 390 and has close to the same HP despite the 390's noticeable advantage in displacement. Depending on how long of a bar you have on the 390, and what you're cutting, you may actually be able to cut just as fast with a 261 wearing 16" bar. So find a 261 for sale, clean the 390 up real good and sell it, then use the proceeds to pay for the 261. There are plenty of people out there willing to pay substantially more for the 390 just because it's "bigger"...
If your cutting needs have you looking for something bigger down the road, pair it with a used saw from the 460 family or the 660 family, or one of their clones if you're willing to turn a wrench on them. Personally, I run an 026 and an 036. I can tell a big difference with the 036, but the newer 261 cuts into that difference noticeably. Enough so that I'd probably skip 60cc saws all together and go straight to a 70cc or larger. The down side to the 70cc saws is that (around here anyway) they hold their value on the used market extremely well. I've frequently seen them bring more than a used 660 in the same condition. Personally, for the extra pound and a half, I'd rather have more power so that I have it when I need it. You don't ever think you're going to need a 36" bar until the day that you do