XT V's are an entirely different animal from SD7s, so that's not a good benchmark to judge by. SD7's are far superior. Stronger, quieter... and XTs generally just suck anyway.
Here's the thing for me. I was running a ceramic rim on the front wheel of a bike that I rode all the time, with an SD7 and a salsa brake booster. (Brake boosters are something to consider in any V brake setup, and salsas are some of the best.) SD7 levers, too.
To date, I'm not entirely sure that the discs have quite topped that setup. I could be wrong, I'm still fiddling with lever adjustments, but so far, I've been thinking that the only difference is an increase in modulation. Granted, ceramic rims will bring the price close to the cost of discs, but with less weight, and comparable performance in rain. I'm assuming you're taking wheels into consideration eitehr way... naturally, bulding your own is cheaper. Just remember never to clean ceramic rims, and don't use ceramic specific pads, since they don't really do as good a job as regular black pads, they just cause more wear. If you're looking to cut costs, use a ceramic just in front, where it'll do most good. If cost isn't as much of an issue, but you think you might see some snow riding, running an alloy rim in the rear is still a good idea. Ceramic rims just don't clear ice as well, and stopping power decreases when the melted snow freezes up.
SD7 on regular aluminum rims still work phenomenally well, in conjunction with the SD levers set for increased leverage. I'm a big guy, who used to ride up and down a hill every day, and I've tried a few braking options. (including XT V brakes... they suck.) SD7 were the first brakes that convinced me that there could be a difference between V brakes... and what a difference.
Recently I've been making a gradual switch to Discs. I run full discs (avid 185) on my XC rig, and I have an avid 203 on the front of my city bike. It's just about broken in, and I learned just yesterday that perhaps having leverage dialed all the way up wasn't really adding power, so much as adding a lot of modulation. I'm still on the fence as to whether the disc brake was actually necessary, since I'm not sure yet that it has really dramatically increased stopping power, and can be more finicky if the rotor isn't quite true. The stopping power thing is pending further experiments with leverage and possibly further bedding in of the pads, though I think they're pretty well done now.
Summary?
They're both phenomenal. But remember in the case of rim brakes that there can be mroe to it thatn just the brake... boosters help a lot, and considering the braking surface (alloy vs ceramic) can yield positive results, too. If you want to go just way too far, build your wheels with disc hubs and ceramic DH rims that can handle disc braking forces. That'll give you the option of swapping later on. Alternatively, you can use a disc brake in the front, and a V brake in the rear... it's a pretty common setup, and it'll allow you to learn about both.
