This is extremely difficult decision in my opinion. Smith is surer bet than Markstrom but has less potential (fantasy) value than Markstrom.

If Markstrom kicks his career into really flying you will regret having to let him go. But for general rule, you shouldn't think more than 3 max 4 years ahead.

I would really evaluate your teams strengths and make a yearly projection for both players what do YOU believe they achieve.

Dont compare them to eachother but compare them to your team and potential team needs in the future. Try to see to next season start (thats not that far away) and think wich one your team needs more in that moment ?

I know i didnt give you straight answer, but this is difficult one.