There is not much difference between the 2, same motor, same plinth, perhaps they fooled around with the arm some more, but I think the RB301 is not as good as the old RB300, just for the fact of the single mounting hole requirement. Anyway, the P3-24 is on sale at many e-stores like the Needle Doctor and Music Direct.
I haven't heard the RP3, so that wouldn't be right to recommend something I haven't heard. I guess if you can get it for the same price.....
I'm not a fan of the Elys cartridges, so I personally would get just the deck and go hunting for a cartridge elsewhere.
I'm sure that there are some cost savings (more profit) for Rega to drag some of the tooling cost reductions to other models. Probably will happen to the P5 too.
Wayner