Which is the Better Tonearm - Pro-Ject or Rega?

You didn't ask, but I can tell you what makes the Audiomods arm so good (especially for the price). Jeff installs internal arm tube stiffeners to get rid of the resonance peaks of the tube of the best Rega arm, also drilling small holes in it in a carefully arranged pattern, also to remove resonances, replaces the steel ball bearings with ceramic ones for lower noise and friction, machines a new one-piece yoke and bearing housing, adjusting the bearings for far lower play than even the best Rega, installs a damped counterweight of his own design, rewires the arm with either Cardas copper or pure silver wire, machines a new height-adjusting turntable mount, with an optional micrometer offered for ultra-fine on-the-fly VTA/SRA adjustment. And there are others! More details on the Audiomods website. The best buy in pickup arms.

I have one and it is great.Makes wonderful music with my denon103r

I am sure the OP is not interested in a $1K+ modded arm.  Going back to comparing the Pro-ject and Rega arms, I have used both types.  The Rega is longer and should have lower tracking error.  The Pro-ject antiskate setup is not that great but then you have no VTA adjustment on the Rega unless you buy shims or custom VTA adjusters for extra $.  Both are a pain when it comes to aligning/swapping carts due to fixed headshells.  If you are considering turntables rather than arms alone, the Pro-jects are a better value for the money.  The Rega TTs are a bit pricey for what they are more so if you need to add a speed control box.  Avoid Edwards TT stuff.

The Rega arms are cast alloy - compared to other arms they are very stiff and by comparison to other materials they are more acoustically inert - that's what makes them better than other arms.

Arms that use alloy or composite tubes, are not as stiff and can be acoustically active (they vibrate)

Jeff at Audiomods has taken the Rega arm tube further and  found a way to make them even stiffer with internal bracing and improved the acoustic damping by drilling holes.

The Audiomods arms only use the Rega arm tube - everything else is all Jeff's design and fabrication.

For the price, they are a great upgrade to a stock Rega arm and mkae a significant difference.

Regards...

Ok, as an engineer, I feel obliged to chime in.  Castings are cheaper than bar or tubing.  Tubing and bar, forged or rolled, have more uniform density than castings. Even if the most advanced casting technics are used, you will still have some internal casting issues, it's the nature of the beast. So maybe Rega uses cast pieces for economic reasons.  Let's not fall for the marketing fluff about castings being more rigid and acoustically inert and all that stuff.  I am willing to bet this claim has never been substantiated.