100+ miles isn't really "long distance touring". Touring is a multiple day activity, carrying with you the clothing etc. that you need. Is that what you are talking about? It's relevant because one of the reasons for choosing spd over spd sl for touring is to minimise the weight and bulk of extra shoes in your luggage.
If you are just talking about single rides of 100 miles or more, then personally I'd go with spd sl or some other road- specific system because I like the big contact area with the pedal. As for the shoes, how much they cost isn't an issue. The fact that you are suffering foot problems might be. If it is an issue with your current spd set-up you should consider either switching to a pedal with a nice big platform to spread the pressure - or going spd sl as you suggest.