Eh, they'll both pull what they were RATED FOR just fine. LS engines were designed to operate most efficiently in a different rev range than the old GenII engines. Just because they need to rev to make peak torque, does not make them inferior to older engines that made peak torque at extremely low revs. I hear that argument a ton. It's a wildly inaccurate myth.
That being said, considering all else (transmission/axle) equal I've had better experiences pulling my car hauler or the family boat with the GenII 5.7L in my rcsb than I've had with an equally equipped 5.3L vehicle. Typically most of my driving (and by default towing) is between central Kentucky and coastal South Carolina, with wildly varying terrain. Anything from 8% grades along I26 and I75 to rolling, two-lane highways more fit for cattle than vehicles. The gearing in a 4L60 seems more suited to the torque curve of the 5.7L, especially when locked in 3rd gear. At 65mph up long mountain grades for example, I can maintain speed and keep the converter locked while in 3rd gear. The 5.3L on the same road would take much more throttle and 2nd gear to maintain speed. Small difference but it's somewhat noticeable. Fuel economy is roughly equal when towing, but my 5.7L will win hands down running with no load. Mainly due to the native lean cruise tables being used in the PCM programming, whereas they're hacked and patched in the LS PCM, so it's not as efficient a system.
