I rarely take Assault Terminators to be fair, but I might as well add an opinion..
I like the idea of taking both of each type of Terminator in one squad... I only tend to take 1 Thunder Hammer in my army though, mainly because of being so similar to a Power Fist, also they don't gain the +1 Attack from having a SS. However, the 3+ Inv benefits them incredibly, going up against tank killers ie. Meltas, Las etc, they are really great at shaking that off. Usually, I take this squad launching from a Raider (Crusader or Redeemer), the hail of bolterfire usually helps them along, alongside the assault cannon.
Personally, I would go with the combination squad, to get used to how they work, then maybe pick up another squad and add the remaining models (ie. You make 2 TH + SS and 3 LC's then pick up another set and make 3 TH + SS and 2 LC's), that way, you can make a complete squad if you want.