Which TT the Pro-Ject RM-5 SE or Rega P3-24?

I suggest the Rega and, properly set up, it should
outperform your Rotel. I'm not a fan of the Project arms
and feel the Rega arm is superior. When your budget allows
you can add the Rega outboard speed controller that makes
the table even better by assuring constant, stable speed.

I agree with Tvad's suggestion you audition the Technics. Its' build quality and speed stability are superior to the Rega. That being said, in my experience, when both are properly set up I prefer the sound of the Rega. It's a personal choice.

Near Mint records, properly cleaned, can sound very quiet. They will never be as quiet as digital. Analog LPs
are played back via a friction based medium. The musical satisfaction, in my opinion far outweighs the flaws. Trashed and dirty records are going to sound awful. A final suggestion, if you are happy with the Project why change? The upgrade treadmill is hard to get off of once you start. A record cleaning machine may be a better use of your money.

Only you can decide which is better. If I were choosing between the Pro-Ject and Rega it would be the latter. However, if you like the current performance of your table the upgrades should only make it better.

Rega actually has very few upgrades for their tables. Most of what you see aftermarket items. With the exception of the external power supply what other upgrades does Rega offer?

Tvad, I've done that more than once. :-)