Headset or Lavalier?

If it is for your own use I would go with a headset (less interference tighter pickup pattern) If it is for a guest speaker I always go with a lav!

Are there any manufacturers where you can change out between lav and headset, using the same transmitter?

Personally, for a 'formal' speaker - I'd use a lav.

For energetic, hope around speakers - I'd go with a headset.

Lavaliers are much less noticable, but if the person is moving around a lot (think Tony Robbins) then a headset would be better.

For the local theatre they're using lav elements taped in place, but they're about to move to headsets (also taped in place).

As was mentioned, nearly all microphones let you swap between various microphones on the body packs.

What's wrong with just giving them a handheld?

I have a lav I generally use it for micing up a priest for ceremony.

Well... I think the folks above have done a very good job covering the bases. As most of you know, I travel a LOT to give presentations and I actually travel with my own mic. I use an "earset" which is the low profile headset, sometimes referred to as a "Countryman". Countryman is a brand, so it's a bit like calling all tissues, "Kleenex". Mine is an EV RE97tx. If I was providing sound for presenters and had to choose only one, I guess a lav is more utility; however, an earset gives you a lot better gain before feedback and is easier to control in my experience. I have frequently borrowed my RE97 to a number of speakers at events I am speaking at. After they hear mine, they ask if they can borrow it rather than using the "provided" mics. So... in a best case scenario.... have both.

Ben