If you know you definitely want a second not too soon after your first hen I'd say its worth paying a little extra to get one that can be adapted into a double. The only thing that would concern me is the size of them, I know they're a lot smaller these days but a single travel system can be a lot smaller than a double. I only really used a double for a few months last year when I had my third (my daughter was 2, almost 3 at the time). I was determined not to use it for too long as I found the size a bit of a pain.