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

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.

Point taken. And nice to here that cautionary note in a world designed to make you want to spend more and more.

This whole thing started with me wanting to upgrade the cartridge and the platter. Plus I was going to get the speed box because everyone says that it helped with pitch stability, and I have already detected this problem with the debut III. Then I realized that it was going to turn into an $800 table after the upgrade, and that's when I thought that what I was looking for was something at that price point.

I have no intentions of becoming an "upgrader" though. I actually hate that aspect of this music equipment thing. I am thinking of the Rega, etc., as a very long term investment (albeit a modest one by audiophile-magazine-standards).

Are you guys saying that my enjoyment of the music might actually decrease with the Rega or the better Pro-Ject?

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.