Why not just keep the SLT1554?
I have one too and I love it. I put 30 hours on mine last summer and have 4 this year. Hasn't given me a single problem except when the mice chewed through my wiring on the starter.
If it only takes 15 minutes to mow, that is a good thing. More time to do other things.
Who is to say that you won't move to a place with a big yard in a couple of years? Then you will be looking for a big tractor again.
If you put only 15 mins a week on the SLT1554, it should last you a long, long time.
Keep it.