I'll attempt to look at this objectively using statistics and another shining example of DLC in the form of Rock Band. As of March, there had been 6 million songs bought via DLC for Rock Band according to Harmonix. No mention is made as to how many were fo reach system, but let's split it the same as the software sales splits go, so about 5/2 360/PS3 ratio. This puts MS at about 4,300,000 songs sold for the 360 version. Now, let's take a couple different hypotheticals, since we don't know exactly how many unique users it is.
First, let's say that number is from an average of 20 songs per person. (An estimation I'd say is quite high.) In that case, it would mean there have been about 200k users actively downloading new content, or about 16-17% of the install base.
If you move it to an average of 15 songs per buyer, then the number of users creeps closer to 300k, or about 25% of the install base.
If the average is more like 10 songs per peson downloading, then you've got 400k people, or 30-33% that download.
So, even in the lowest scenario, percentage-wise, that leaves 1 in 6 buyers that actively download content. If I had to guess, I'd guess the more realistic ones would probably be closer to the 20-25% range. That's not a terrible percentage of people aware of the availability of DLC. Given that Rock Band is what I'd consider more of a "casual" game than GTA, I'd guess the more "hardcore" gamers buying GTA would be even more aware of its availability. To think, as some have insinuated, that it would only move a hundred thousand units or so, would basically mean that it only sold half a million copies, which even the most undying Sony fan would have to admit is a ridiculous propostition.
The 360 version will likely outdistance the PS3 version's sales. Not because of DLC, but because of its increased install base in North America and the rest of the world outside Japan, where GTA is a non-factor. The game is a few weeks away, and the DLC is still months beyond that, so ths still has a long way to go before it really plays itself out.