In September last year, I traded our '99 Burb in on a '05 Deisel X. I bought the Burb used but had it almost 4 years. Reliability was OK until we got close to 100K, then everything the GM coolant touched started to go, got expensive and very frustrating to get the calls from my wife. When the fuel pump (top of the tank, no access) started to act up, I traded it that weekend. I know, mine was the older body style than what you are looking at, and can't speak for the 5.3 engine as mine was the (faithful?) 350ci 5.7 - so your results may be different.

However, before trading, I looked at and drove a couple 02-04 Burbs or Yukon XL's and made direct compare to the X, other than towing which most test drives don't allow. The things I think the Burbs do better are: 1) in-town road comfort, softer ride on crappy roads, but the stiffness of the X is not as painfull on the hiway, and appreciated if towing. 2) Front seat comfort is a little better, not huge fan of the buckets in my 2005 EB model, mostly the way the bottom of the seats pushes inward on my legs, and should be 'deeper.' Even my '99 burb seats were more comfy, but quality of that leather was horrible. Also, only 8k miles, so this may change w/ breakin.

Everything else, the X does much better. Too much to list of couse, but look at the interior room. We wanted the 4-capt chair option, and in the Burb with that option, you had to squeeze between them to get the the aft seat. In the X, it's comparable to most conversion van seating, not cramped at all. Hiway handling and power (V 10 or esp 6.0 dsl) is much better. I went for the deisel b/c of extra power in case I'm towing a larger trailer in a few years. Very glad I did, I'm getting 16 - 17 mpg comuting on mostly 2 lane roads, a friend with a '05 tahoe (lighter than the burb) gets 14 avg on similar comute. Ouch. Almost forgot, the brakes were awful on our burb, and not any better on the newer ones I test-drove. Spongy pedal and stressfull if you had to make a quick stop -- 100% improved on the X, braking is much more confident at "car-like" to quote my wife.

My opinion, only a tough choice if you aren't towing heavy things and if you like the softer ride. If you happen to get the GM, change out that pink coolant they use ASAP. I wish I had.
Good Luck!