Triathlon - General crankset question

What is the difference between a 54/42T crankset and a 53/39T crankset? I know it has something to do with the number of "teeth" on the cranks right?

Also I am looking at two tribikes - one has a FSA Gossamer TT Mega EXO aluminum crankset with the 54/42T set and the other has an Ultegra 53/39T. Obviously the Ultegra is better but how much better? I am pretty big (6'4'' - 200 - 210 lbs depending on the season) Would I wreck the aluminum set when I really crank down on it. My road bike is all Ultegra and I never have a problem.

the difference is in gearing. Given the same cassette, the 54/42 will be harder to push but you will go faster at a given cadence and gearing. your average speed should be about the same however. climbing will be slightly more difficult with the 54/42 (the small number is 3 teeth bigger) as far as the ultegra being better, i would say not much better. no you will not wreck anything. those cranks are rated to a 400 lb guy with a higher power output than anyone at the TdF.

The FSA is a boat anchor. Crank + Bottom Bracket weigh in at over 1000 grams.

Those numbers are literally the numbers of teeth on the large and the small ring. A full double at your local LBS is typically 53/39, and compact double is 50/34. Full has larger rings/more teeth and gives you higher gearing for a given position on the cassette. Compact has smaller rings/fewer teeth and is more suitable for weaker/older riders, or those who like to climb but do not want a triple.

This is all relative. Gearing, pedal force, cadence and speed all interact with one another. Here's something that will help visualize what is going on:

http://www.mne.psu.edu/lamancusa/ProdDiss/Bicycle/BIKEcalc1.HTM

EDIT: oh yeah Ultegra should be better and quite a bit lighter. FSAs are HEAVY and they have a tendency to blow the BB seals or start to tick after about 1000 miles. Ugh that's annoying. I'd go with an Ultegra.

You may want to consider a compact chainset as you will probably find riding at higher cadence( 90 rpm +) easier than trying to turn a big front chain ring .
Also if you are a runner becoming a cyclist you may put too much strain on your knees trying to power it out at lower cadence.
you maybe fit enough to do it and that could be the problem ...ie fit enough to turn it but without the bio mechanics that thousands of miles develop that makes low cadence diesel engine 85 rpm on a 55t front end possible without giving yourself patella tendonitis

I would vote for the Ultegra 53/39 from those two (if we are voting :-) ). Personally, I love a compact (50/34), and find that it gives me all the high as well as low end I need. Unless the FSA has hollow crank arms, the Ultegra should be stiffer.