I got my S9+ for two reasons:
1) I had decided to get the latest Galaxy S, and the + had a bigger battery; and
2) I had to get a new phone as the old one broke, and it was right in the middle of the Costco sale on the Galaxy S9 series.
I had a Blackberry KEYone before, and it was a laggard with poor performance. I said screw it, as long as I have to, I'm getting the S9+.
No real regrets; as a pocket computer, it's very very nice. VERY nice. I do like some of the Android 8 features compared to Android 7.
But as a phone, it sucks. My voice keeps getting muffled and cut off as I try to talk to other people. No doubt this is due to T-Mobile's decision to have Samsung take away the ability to turn off HD voice and the ability to turn off noise reduction.
Lewis Black sums it up generically:
https://www.youtube.com/watch?v=No8tIAsBYjc
Anyway: one reason to upgrade, especially in the T-Mobile world, is to keep up with the new frequencies that are coming out. The S9 series has everything today, but the S10 will have more--and that's coming in March.
Can you wait until March?