Speaking as a former salesperson turned business owner I will tell you that I firmly believe the only way to go is straight commission. Typically serious salespeople don't like salary + commission because the commission is lower than if it was a straight commission position and they make more on straight commission (I always did). I tried out different pay plans for my salespeople and I got, by far, the best results and best return on my money by doing a generous straight commission.
I can say this since I used to be one, salespeople can be very lazy. Give them a salary and they will typically just sit back and rely on that. I've known some real big-hitter sales people and everyone of them would only work on straight commission because they made more, they liked the competition aspect of it, and it drove them to do more and be better. (Competition is a huge motivator for a salesperson!)
Also, if you go straight commission you can pick up some great part-timers. My part-timers aren't throwing up outrageous numbers or anything, but they are consistently doing good work and are a little less maintencance than my full-timers (which is to be expected since this is a full-timers bread and butter).
Of course you can do whatever you want, but I would strongly encourage you to do a straight commission that is as generous as you can be. Now, I can't speak with 100% certainty without knowing more about your business, but that is always my initial response whenever I get asked this question. Feel free to PM me if you want to talk further about it in greater detail and I'll try to give you the best advice I can!
Oh, and as for not being guaranteed payment if you're straight commission: all I have to say is that any good salesperson that is worth working with knows that they are guaranteed payment on straight commission because they know that they can get the job done. Those are the people you want to be working with, not someone who needs a salary because they don't know if they'll be able to actually get the job done. A true salesperson should know their closing rate and so it's very simple for them to calculate out how much they'll be making.