E:TW is a wonderful game, and I have thoroughly enjoyed playing it. The criticisms which are often expressed on this forum should not be taken to mean that the game is dreadful or in any way unplayable. Rather, many of these criticisms are expressed by veteran players and TW enthusiasts who are justifiably disappointed with the apparent absence of features which should have been included upon the initial release. Many of the initial issues have since been rectified by the post-release patches, while still others may be fixed in the near future.
I heartily recommend either. Both are fantastic games, situated in vastly different and exciting periods. The graphics are comparable, although E:TW does include landscapes, textures and animations which are superior to those of M2:TW. This might be of some importance depending on the processing power of your computer system. There are many gameplay differences as well, which may incline you towards one or another. M2:TW maintains many features which are absent in E:TW, while lacking some of the innovations of E:TW. There have been a number of mods released for M2:TW and many others in the making which may incline you to buy it rather than E:TW which has very few at the moment. The initial release problems with M2:TW have been fixed as well. Additionally, M2:TW is far cheaper than E:TW, costing sometimes >15$ for the Gold edition which contains the highly enjoyable expansion.
With all of these considerations, I would suggest that you invest in M2:TW first as it is far cheaper and remains a wonderful game. You will surely enjoy playing it for dozens of hours. Once many more of the patches and modifications are released for E:TW, it will in a better condition for you to play the Grand Campaign. However, the Naval Battles in E:TW are an epic feature lacking in M2:TW, and this may incline you to purchase it.