I really enjoyed the 2100 that I had for a time. To be honest, it was a little tricky to get parts for, but it was a lot of fun to have the beast around. Besides, NOTHING in todays line-up sounds quite like it.
One of the easiest saws to tear down, clean, fix, and put back together, problem is though, that whole fix thing. If I got one again, I would definately buy one of those "lot of nuts and bolts" from a 2100 that come up from time to time on the Bay. Have fun with it, again, it's a lot of fun, but I think your 660 will still cut noticeably faster just due to the higher RPM's.
The 288xp on the other hand. I still use one as an everyday saw. Sure I run a 7900, but I keep a 28" skip on my 288, and it takes care of many a beech and sugar maple trunk round here. For me, the 288 has been the hurkiest saw I own. It has been so good to me, that I didn't buy a 660 when the time came to decide, and I bought my 088 instead. Granted the 088 only comes out once in a while, but that 288 comes out all the time, and won't die.
I can understand why you love those 2 saws, I do too. And I don't have a favorite brand, I own a little of a lot of brands, and they all have their place. Enjoy you 2101, and the future 288.
Jason