In terms of lumens/watt, the LuxV is probably the best there is when looking at the 'high power' leds. It is way behind some of the smaller 5mm leds however, but they won't really be of any use for you.
The LuxV is also a winner in terms of lumens/size - it throws out a lot of light from a small package. But they are relatively expensive in terms of lumens/dollar, so you might want to re-think the idea of using a couple. Basically what are you going to do with them? If you need a light to throw well then a Lux3 would be better as they will focus better, and if space is not a concern then 2/3 Lux3s will be cheaper (but take up more room and be less efficient).
As for voltage drop - that depends on the exact leds you get. They are 'binned', or separated into groups of leds that have about the same voltage drop. You need to know the bin of the leds you want to use. LuxVs range from about 6v to 7.5v or so, Lux3s from 3v-4v. You really want to drive them with a constant current source though (not constant voltage), as the Vf is likely to drop over time, which will put up the drive current and hence cook the led if you use constant voltage. By using constant current drive it doesn't matter so much what the Vf is.
Hope this makes sense. If you let us know more what you want to do with the leds then we can try and give more specific help.
And welcome to CPF!