To be honest you could probably sell the Alfine and fund both wheels. But if you’re planning on keeping it, I think your choice depends on what type of terrain you ride. My route to work is 25 miles of fairly hilly roads and its the down hills that kill me. Nothing too steep to make me feel like I’m in too high a gear, but on the other side its chuffing hard work and you don’t get any time to recover before hitting the next hill. Graft!
I ran an SS bike last winter and very much enjoyed it, but feel i get more of a work out on the fixed. That said, I think the fixed is actually easier than riding a geared bike to and from work – with gears I tend to push myself harder where as with fixed you just sit there and spin on the flat bits. Once you’re used to it and worked out that you actually free-wheel a lot more than you think you do it becomes quite enjoyable. I’d always planned to flip the wheel and run it SS when I bought the bike, but so far I’m enjoying it too much to bother