well, IMHO
you use lavalier when (when using sound on tape / sync):
- you can't control framing
- you are one man team
- you have a wide shot you have no place to hide boom
- indoors in reverberant space, where close placed omni gives you the better result
as for what to buy, things to note:
- legal (free) frequencies
- power requirements
- get the best lavalier you can get (Countryman, Sennheiser, DPA, Rode)
- get the best radio system you can afford (Sennhaiser, Sony, Rode)
to conclude- you need both boom and lavalier, and a radio system, there are ENG type kits, that have one receiver, one belt-pack transmitter, one block transmitter, or get two kits from rode one newshoter and other filmmaker. Rode is one of the best band for the buck, built well sounds good. And 2.4GHz legal anywhere, not like 400 to 860MHz, you have to now your why around.