It depends on you and your players. I like both. Read my description of the strengths and weaknesses of each and decide what you and your players enjoy the most.
SPOILERS BELOW!
Strengths of TOEE include the Village of Hommlet and Nulb, the broken tower bandits and the leaders of the tower on the 3rd level. These areas are well-suited to running dozens of side treks and I've easily created a full notebook of ideas of ways in which the PCs could adventure in those enclaves and the surrounding area (e.g. the bandits of the tower of the TOEE raid Hommlet's jeweler and the PCs are asked to form a posse with some of the badgers and npcs and hunt them down). I also loved the machinations between the various subtemples that is so well-illustrated in the CPRG that its almost worth buying just for that reason. The original TOEE has amazing potential for PCs to connive to their hearts content between the various factions, the troll chief, the various power centers in Hommlet and Nulb, etc.
On the weak side of TOEE, I found the Greater Temple and Elemental Nodes climax anticlimactic. Talk to the guys on the Dragonsfoot website about what Gygax intended to do and it is quite different from the actual product. I'd change part of the third and most of the fourth level of TOEE if I was dming it. It just isnt as interesting to me as the first two-thirds of the adventure.
Turning to Night Below, like another poster, I found the First Book of Night Below to be decent (particularly the start) and the Third Book of Night Below to be awesome (particularly if you buy Lords of Madness you could make Shabaoth a cool place!). On the weak side, Night Below can get pretty repetitious -- there is a reason so many previous posters only played two-thirds of it, etc. I thought the middle part with the endless trekking underground in the second book wasnt very strong and could probably be cut by 70% or so. In addition, in the third book, I loved Shabaoth, but I thought that the task of destroying all three towers got a little repetitious as well. Like a previous poster, Id strongly suggest adding a totally unrelated adventure in place of a good part of the Second Book, preferably outside or in a city or on another plane just for a change of scenery. In the third book, Id also suggest changing the number of towers to just one to make storming the tower a true climax.
In summary, both modules need some work by the dm to avoid defects. Both modules are pretty good about providing opportunities to engage evil factions diplomatically so there is no real edge there. TOEE has much better and more memorable npcs (and names of npcs!) and the npcs really come to life with the CPRG if you listen to the sniveling whining voices of some of the temple priests, as well as much better town settings allowing for more social interaction so if you and your players value that, you should lean towards TOEE. On the other hand, I thought the endgame for Night Below was much more satisfying (and creepier in a Lovecraftian way) than TOEE. Therefore, if you like Cthulhu-esque sorts of things, you could really go to town with Night Below in a way that you couldnt with TOEE.
Hope this helps
