the BD bike would be a much better and probably cheaper bike in the long run. I have done conversions as have many of my friends, and most conversions involve at least one major headache. You will end up having to replace more parts than you realize, and I'd be surprised if you could complete a conversion for under $100. A new wheelset alone costs at least $100 (cheap), and rear wheels on their own can be hard to come by. Then you have to buy cog, lockring, tires, tubes, etc. you also have to get a new chain and then replace/fix any other parts of the bike that need fixing (brake pads, etc). All this for a crappy, hi-ten frame.

Buying a used conversion from someone else can also be problamatic. You don't know who built it or with what parts. Something could be broken, threads could be stripped, etc. Now, I have bought several bikes off CL but I feel like I know what to look for. Unless you feel comfortable in your evaluation skills, I wouldn't do it.

The BD bike may not look as cool, but it will give you much less problems. Also, for a first FG bike, you should just get something that can get you out there and riding ASAP. Save the build for your next bike...