According to Rega, the
RB301 is an improved 300, but I don't think they sell it separately from the P3-24.
There's no difference between a Moth and an OEM Rega. Rega supplies their arms to a number of manufacturers and allows them to put their name on it. The Michell Tecnoarm is a different story. It's a modified RB250 that's significantly better than an OEM. Read about it
here.
The standard RB300 and RB250 are nice arms that can't be beat for the price. Either one would be an easy drop in without changing the mounting hole. There's a lot to be said for that if you haven't mounted an arm before. I agree with others to stick with Rega. There are lots of tweaks and upgrades that can be done to the 250 and 300. Michell makes an excellent counterweight called the TecnoWeight. It's a significant upgrade that improves the low end. They also makes a VTA adjustment sleeve that works well. I've never seen the VPI version Stanwal mentions.
An Origin Live is unquestionably better than an RB300 or 301, but you'll pay 3 or 4 times as much.
If you own the Grado Platinum, live with it for a while. There are plenty of alternatives when the upgrade itch hits. Focus on getting the table together first.
-Mark