I'm fitting out a new frame I got for Christmas with many of the parts from my old bike.
2003 Allez Elite Triple moving to Allez Comp. I couldn't use my old 105 front derailleur as it was a clamp on and the new frame is a braze on. So, I bought a new 105 triple for the front. The old 105 handled a 52, 42, 30. The new one cannot handle that span of 22 teeth, and requires at least an 11 tooth difference between the big and middle rings. So it's either buying another derailleur or getting two new chain rings to have a 50, 39, 30.

With that said, what is the difference (besides price) between the 50, and 42 rings for a double and those for a triple front?

If this has been answered before, please tell what to search for, or post a link to the the discussion.

Triple middle rings have ramps and pins to assist lifting the chain from the granny gear.
Compare the width of the derailleur across the inside of the rails. The difference in chain width may be a problem.
There have been inconsistent reports on how well a 10-speed triple front derailleur works with a 10-speed drive train.
You could always try it and see.
I think the 9-speed triple FD is more forgiving to incompatibilities. If the 10-speed triple FD doesn't work well try to find a Shimano 9-speed braze-on type FD, Ultegra, 105, or Tiagra should work.

So, by inference, the 50t big ring would not have much of a difference, but the middle is more significant. I've got a 9 speed in the back--how does this help or hurt?

There is also a difference in synchronization but I think that is not a big problem. The 9-speed cassette is not an issue, the chain width may be an issue. The 9-speed chain is wider, the 10-speed front derailleur is probably a bit narrower than the 9-speed FD.
Since you already have the 10-speed triple FD, why not try it. If you have problems you could try a new 10-speed chain, I think it would work OK with the old cassette and crankset. But try the old cassette, chain, and crankset first.

On one of my bikes I am running a Shimano front derailleur designed for 50/39/30 rings but with 52/42/30 rings. No problems.

On another of my bikes I am running a Shimano front derailleur designed for 50/39/30 rings but with 50/39/28 rings. No problems.

If you're feeling experimental, you may find your existing components interoperate just fine.

musterion, Two things, first make sure that the clamp-on FD isn't a braze-on FD in an adapter clamp. If it isn't, try the new FD.

Brad

bradtx: No it definitely was a clamp on. It was the "stock" FD on the bike.

ryker: How I noticed this problem was when I was fitting the FD with the sticky that comes on it, I noticed that the inner part of the cage would not clear the teeth of the middle gear. Do you have your FD set a bit high on the 52/42/30 so the inner cage clears? How would this affect shifting?

It slows down actuation into each ring, the higher the derailleur is from the optimum low position. That being said, i've seen some ridiciculous amount of gaps before with no loss of front derailleur performance.

YMMV.

different a couple of grams in weight maybe...