Welcome aboard, Peter.
Your questions:
1. The soldiers are not only in the intro, but in the game as well, but do not expect them to be of uber-AI.
2. SF certainly is the better choice over TvT, I think most would agree on this now. But be advised that the publisher of both sims, Lighthouse, has gone bancrupt. Which means - no further support for their products from them. Mods try to make something of both things, but so far I have seen sims being much more active in modding. Do not expect wonders. Take wonders, if they ever come true, as an unexpected gift.
But somebody here posted about a random mission generator he has developed and is testing. Such a thing usually comes highly welcomed by any community.
3. Whether or not SF is, as you put it, a "hardcore" sim, has been object of bitter and angry debate in the past. Some say yes, I say no, but I also say it is a solid game if not trying to see it beyond it's limited scale. If you want "hardcore" tanking, there is no alternative to SBP.