I installed new floor so the transition height has changed from one room to the next. I took off the baseboards first before installing the floor. The problem is I am not sure what's the cleaner, nicer way to combine baseboard and transition. Do I put baseboards first then try my best to get the transition piece the perfect length or do I put transition piece first and then cut a notch/es on the baseboard so the transition looks like it goes into the baseboard?
I would imagine it's a matter of preference, but IMO the transition is part of the floor and the baseboard is part of the wall. Given that the transition will blend in with the floor, I'd be inclined to put the baseboards on first and then cut the transition to fit. It's also generally simpler to cut something to length than it is to notch something cleanly and precisely.
Ultimately I don't think there are any severe consequences for doing it one way rather than another.
I like baseboard last, but keep the top edge the same height (don’t step it up or down at the transition).
If you install transition last, you’ll be able to look down directly at the crack between the transition strip and the baseboard. If the baseboard is installed last, it will cover the joint when looking down.