If you can test one out first, I might suggest that. It is certainly true that a box mod is more flexible.
Fixed wattage setups can be somewhat hard to mix and max different tanks. If you love it, great, but if you don't it is certainly true that it will be harder to find another suitable tank. So if there's any way to physically get your hands on one, I would try that. Even if you've vaped 60 watts, that's not necessarily a guarantee that you will love both the mod and the tank.
I've never owned a smok product so can't comment on their quality, but I will comment that for probably not that much more, you could get a more flexible setup. With one reliable box mod you can experiment with a lot of tanks. With that also said, if you don't dislike the idea going in and are prepared that it may not be fantastic for you well, then you have that going for the kit. Usually though, by the time sub0hming 60 watts on one battery, well, the battery life may not be that fantastic.
I've had one stick mod that was fixed, and it was an aspire tube. I didn't much care for it, honestly, although I was still dual using at the time, so that may have colored my perceptions, and I LIKE aspire products.
Anna