From everything I have heard, the Moth and Rega arms are identical on a model for model basis. Rega supplies OEM arms to many of the other TT makers as well.
Regarding the arm upgrade to your P3, are you considering an RB 900, as it sounds like in your post? If so, in my opinion, that arm is overkill for a P3. Considering the stock RB300 on your P3 is about a $350 retail arm, that leaves you with about a $300 TT. The RB900 is a $900 retail arm. Do you think it makes sense to put a $900 arm on a $300 TT? The RB300 is a very good arm, and the RB900 is really only an enhanced RB300. Most people would be more likely to spend the $900 on a better TT, and transfer the RB300 over to it. I might suggest a nice used Linn Sondek LP12 or a used Sota Sapphire, which would fit nicely under that price, and sound way better than the P3. They will both take the RB300 transferred onto them very well. That would give you alot more sound improvement for your money.
However, If you really want the RB900, you will have a very nice arm on your P3.
Regarding the arm upgrade to your P3, are you considering an RB 900, as it sounds like in your post? If so, in my opinion, that arm is overkill for a P3. Considering the stock RB300 on your P3 is about a $350 retail arm, that leaves you with about a $300 TT. The RB900 is a $900 retail arm. Do you think it makes sense to put a $900 arm on a $300 TT? The RB300 is a very good arm, and the RB900 is really only an enhanced RB300. Most people would be more likely to spend the $900 on a better TT, and transfer the RB300 over to it. I might suggest a nice used Linn Sondek LP12 or a used Sota Sapphire, which would fit nicely under that price, and sound way better than the P3. They will both take the RB300 transferred onto them very well. That would give you alot more sound improvement for your money.
However, If you really want the RB900, you will have a very nice arm on your P3.