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Dual stage oil pumps will achieve a better vacuum under ideal operating conditions (which composite work isn't!).
Dual stage oil pumps have two pumping units connected to the same motor. The exhaust of the first stage is connected to the inlet of the second. This will produce a lower vacuum if they run for a while. But since the ultimately achievable vacuum in vac bagging etc. is dictated by gas permeability of the bagging materials and tubing and the vapour pressure of resin components they do not necessarily produce a significantly lower (better) vacuum under real world conditions. Single stage oil pump might be sufficient, but it can't hurt to use a dual stage pump. They are of course a little pricier.
Membrane pumps are available as single, dual and even multiple stage pumps. Single stage pumps will only produce a vacuum of approx. 80-100mbar or so, which is insufficient for most of our purposes. Dual stage membrane pumps, well it depends on how the pumping heads are connected to each other. If they are in parallel, they will produce the vacuum twice as fast and will move roughly twice the amount of air per minute, but still achieve only a weak vacuum. If they are connected serially, then they will produce a better vacuum (up to 15-20 mbar) but will move just as much air per minute as a single stage pump. Then there are membrane pumps with multiple stages, where all the pumping heads are connected serially, they will go down to 2-3mbar under optimum conditions. These are rarely found outside professional laboratory environments. The reason they are used is that they can be produced out of fluorinated polymers (think teflon) which makes them insensitive to volatile solvents and agressive chemicals, which would quickly ruin oil pumps. They are also quite expensive, with prices about 10-30 times higher than one of those el cheapo Chinese Ebay oil pumps.
Then there are hybrid pumps which consist of an oil pump connected to a membrane pump, but mere mortals will probably never see one in action, they can only be found in well-equipped and funded professional chemistry labs.
If you are on a budget, a small single stage oil pump will do for smaller scale work. If a few bucks more won't matter then buying a dual stage pump can't hurt. But what's more important is the amount of air the pump is able to move, the cubic feet per minute or cubic meter per hour rating. The higher this number the faster will the pump achieve its ultimate vacuum. Really big projects require a big pump. The two models EC offers are actually good compromises, from the pictures I have seen I assume that they are both dual stage pumps. The small one will work for most applications, the bigger one is only necessary if you are in a hurry degassing fast resins or work on really big projects.
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