I too much prefer 3rd person in general - of course there are some noteable exceptions.
I find that I am not more immersed in 1st person games and if anything generally just the opposite.
I do not suffer from any motion sickness.
But I think the reason is that in 1st person games I am continuously getting frustrated with navigation - all that east, west, north, south, 3 steps here then turn, 2 steps there, 4 steps to the right, forward, back, etc.