My experience was the opposite of Jeff's: I had some higher expectations of Split, after reading Rick's suggestion that it was some sort of bustling cosmopolitan city, not at all touristy like Dubrovnik or elsewhere in Dalmatia. I spent two nights in Split before departing from the Split airport out of Croatia. I was unlucky enough to arrive mid-October as the weather seemed to be turning rainy for the season. The first night it was rainy and windy - not a surprise that there was no one out for the evening. But the second night was much nicer and still the town did not seem bustling or popping with cafe life or anything. (I'm sure there are enough bars and cafes within Diocletian's Palace, but I didn't spend a lot of time out that late.)
I would call Split interesting, but it did not really meet my expectations. I didn't hate it, but one night there would have been more than enough for me. I imagine in the summer it is very different on the Riva on a warm evening, probably full of life and crowded. So I suppose the feeling of the place depends when you visit. It certainly wasn't crowded in mid-October.
I did not visit Trogir to compare it.