Carbon vs. Aluminum frame bikes and tri lengths

Carbon bikes are likely lighter, yes, but that shouldn't really be your overriding decision characteristic (unless you're in the neighborhood of 4% BF).

Carbon bikes make the ride much smoother.  Thus enabling you to go longer distances and feeling better.

When I was shopping for my tri bike, I rode a P2SL and a P2C, back to back on the same road.  The P2SL was decent but, when I rode on the P2C, it was leaps and bounds more comfortable.  The road felt so much smoother.  I didn't want to get off the bike when I was done.

Actually it's all a bunch of bike mythology and heresy ........

You can make any frame material react in just about any way you want.  You can have a whippy and flexy alumnium just as easily as you can have a super stiff and responsive carbon.  It entirely depends on how the manufacturer designs and builds the frame.  And weight is not any different in that equation.  Cannondale got busted a few years ago with the Saeco because their bikes were below the UCI legal weight, so they had to make them heavier ...... and they were aluminum.

It also depends on which components you put on the frame.  Some absorb shock better than others (fork, bars, stem, seatpost, etc.).

To beat a dead horse, it is all about fit and comfort for you as to which one you decide on.  That and your budget .......