I was going to get the Core 55, I went with RJ60 instead. Roughly the same price. Both overkill for my 18k pool. I just got pool resurfaced, replaced the pump, new filter, and added the SWCG. I just got the RJ installed yesterday (plumbed, but not powered yet).
The Core used to have 4 power settings, now it's at 6, so that is a plus. And it definitely looks easier to install. Takes up less front/back room, but more up/down room. No flow-meter, that's built in.
I ended up with the RJ because it's control box has more info, it'll tell you salt level, power usage, amps, I think water temp, a few other things. Even if not perfectly accurate. I like the numbers better than just having an indicator light. But it was a tough choice between data and convenience, especially with my compact setup.