To me (as someone with top tier status in Marriott, Hilton, and SPG), it comes down to two things:
1) For the places you're going to be travelling, are their good options to stay? If only have the places have a SPG hotel you aren't going to be getting status or points as fast
2) Pick a couple places you might want to redeem and see what the options are. There are some locations that are very under-served by SPG (England is pretty bad, especially outside of London; though I realize this probably doesn't qualify as 'exotic').