I'll start off the discussion by saying I normally play in the 500-1k PV range. In the beginning, I mostly focused on mechs because that is what I had from the Clan Invasion KS. I've got to say though, that I really enjoyed the games where we included BattleArmor. I'd be open to including regular infantry, but haven't gotten around to it yet.
Since we regularly play some form of objectives, BA and Infantry would fit nicely as holding units. I've used Elementals when playing Clan, and also some Grenadiers when playing as IS. Obviously, Elementals flexibility with Mechs as battle taxis is really nice. I've been looking at the Infiltrator Mk II (Magnetic) so I can use them with my standard non-omni IS forces.