I think the decision to go from DW to crank is all about volume and time. If you are only stringing your racquets, and you only have 2-3 racquets, a DW should be more than enough. If you plan to string for others at a rate of 7+ a week, you may want to jump up to the crank.
However, time is a crucial factor as well. If you only string for yourself, but have very little time to devote to stringing, a crank might be right, in spite of the limited volume. If you string for others, but have all the time in the world, a drop-weight may be sufficient.
Also, keep in mind that just because you string your racquet once a week now, doesn't mean you will string it once a week when you have a machine of your own. Stringing is addictive. You will more than likely find yourself, initially, cutting strings out before you normally would.
Also, keep in mind that you may find that you like stringing enough to string for other people, in spite of initial feelings against the idea. If you find that you wish to string more, but don't have the time to give to a drop-weight, you may regret not getting a crank.
All in all, it's a personal decision. The quality of string job can be just as high with either type of machine, so it's down to, like I said earlier, volume and time. Oh, and money plays a factor, of course. But that needs no explaining.