I have never played MTW, so I'm not sure how RTW appears to an MTW vet. Coming from a sort-of general interest in strategy background, I have been delighted with RTW. The battles were rather challenging at first. I now win about 90% of them on the "hard" setting, but find that the morale bonus given to the AI on "very hard" simply serves to make the battles difficult in unrealistic ways.
The campaign game is interesting and seems to play out quite a bit differently with each game. Factions that disappear early in one of my games sometimes dominate in another. There are exceptions (Egypt is always a power), but gameplay does seem to be different enough to make each campaign interesting.
I am extremely happy with RTW and highly recommend it. I've put in way more hours than I would have considered sufficient to offset what I paid for the game. Heck, I've probably played about twenty hours a week for the six weeks in which I've owned it.
I will probably give MTW a try at some point, although I am disappointed to hear that the position of armies on the campaign map when attacked does not translate to the battle map with the same precision that it does in RTW. This seems to me to be a key part of the game and I would miss it quite a bit.
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