yeah, depends on your usage. If you're exclusively pumping or going back to work while still BFing then you will probably want a double.
Honestly, I don't know if I'd buy one before your child is here if you can help it. I thought I would exclusively BF but it took us weeks to get the latch correct. I rented a pump for the first couple months then could have moved to exclusively BFing but I was going back to work so I bought a double pump.
If you bought the single and ended up in the situation I was in, you might regret spending the money.
I say, research the ones you like so you know what you want to get for different scenarios, then buy when you actually need it since you don't know what circumstances will be like. If breastpumps were re-sellable or returnable I would say go for it now.
Pumping usually takes 15minutes of actual pump time. So using a single that is 30 minutes of pumping. A double would only be 15 minutes.
Again, all depends on your usage. If you are going to feed on one side and pump on the other, a single is fine. If you're only going to pump once a day or less, a single is usually fine.