I'm not that familiar with the Miles & Smiles program, but I'd check first to make sure you can actually use the miles for hotel stays. I didn't see anything on their site to suggest that, but I just looked briefly.

M&S miles expire three years after they're earned, regardless of any intervening account activity. Thus, if you were trying to save them up for an award, you'd have to be careful that they didn't expire. MileagePlus miles expire after 18 months of account inactivity, but there are inexpensive ways to keep your account active even without flying. So, I'd argue that it's easier to accrue a balance in MileagePlus -- but if you're certain you'd use the miles without 3 years of your travel, then M&S could make sense.