What are you planning on cutting? What length bar do you anticipate running most of the time? What is the longest bar you'll ever want to run?
Having used both saws for firewood, storm cleanup, and to make money, here's my take:
1. MS260: it needs its muffler opened up. Badly. Once opened up and retuned, it is a strong little saw. The emphasis here being on little saw. Keep a 16" bar on it and you'll be happy. Oregon 20LP is the chain to use, as it cuts a slightly narrower kerf than Stihl RS, which in every other respect is a better chain. But the performance gain with 20LP, coupled with its ease of hand sharpening, is well worth it. If you intend to run a longer bar on the 260, which I would not encourage, then the adjustable oiler would be nice; I don't have the specs with me, but I think its maximum output is greater than the fixed oiler on the 260 non-Pro. If you're running a 16" bar only, then save your money - you don't need the PRO's adjustable oiler. Under no circumstance is the decomp necessary.
2. MS290: it is plasticy, it is overweight for its power, and it is somewhat more difficult to rebuild (but not that much harder, really) than the vertically-split crankcase "pro" saws. That said, it is a dead reliable design with a very low failure/service rate. Having owned both (026 and 290) at the same time, I can say without question that the 290 has more torque and can honestly pull a 20" bar better than the 260. It, too, is at its best with a 16" or 18" bar, in terms of balance and performance. The lower revving, higher torque design tends to work better with wider-kerf chain like Stihl RS, though Oregon 20LP would work fine on this saw, too. The 290 also benefits a lot from having its muffler opened up.
Given the choice between the two saws, I'd honestly go with neither and get an MS280. As a firewood saw, I think it has a lot going for it - more power than the 260, a wider/lower powerband than the 260 but peppier than the 290, and it is much lighter weight and more nimble than the 290. Price-wise it's right in the middle, too.
In the interest of full disclosure, I ended up selling my 290 and my 026, and for my "middleweight" saw I have been running a rebuilt (soon to be woods ported!!) Husqvarna 154 made back in 1984. It outruns both of the Stihls, weighs and handles like the 026, and actually cuts side-by-side with a broken-in Dolmar 5100.