GM seals, hands down, any day of the week.
You can be cheap and deal with the consequences, or buy GM and be happy and expect 20 more years of service from them.
I've had LMC and GM seals, if GM is still selling what you need, buy them now (and more than one set if like me you will never get rid of the truck) before they quit. gmpartsdirect.com is where you need to buy them from, the dealer will rape you.
There is a post in the tech forum that is titled "post your part numbers" or something like that, I know I've posted a few seal part numbers there. Good time to get door seals too if you know they are needed.
I know for a fact you cannot get the upper rear window seal (the one that seals the rear window to the topper, u-shaped) from GM anymore, and the LMC piece was a piece of junk when I bought it. Poor quality all around, and not even constructed correctly.
Tailgate diagram/part number page here will get you three tailgate seal part numbers:
http://dyeager535.topcities.com/partsmanual.html