The longer the stroke on a motor the more it is inclined to torque rather than revs. The 4.6 has a longer stroke of 3.2inches compared to the 4.2 of 3.03inches. Standard the 4.2 makes 200 hp and 251 ftlb of torque, whereas the 4.6 makes 225HP and 280 ftlb of torque.I also have a 4.2L that could use a refresh/rebuild (stock rebuild, perhaps a better oil pump and a Crower cam?) that I was planning on dropping in my RRC sometime in the future. Stock the 4.2 are supposed to make 200hp and with hot cam, the one I'm looking at claims a good torque increase.
So if you are after power and torque, the answer is obvious.
I have never had a 4.2, but I run 4.6's in a disco 1 and Rangie classic and they supply good power and torque. The earlier 4.6's in the mid to late 90's had issues with the block, but this was largely fixed in the later models. So try and find a motor that was made say 2000 or later.