Dish clearly has the advantage over DirecTV for price and hardware. With 2 different HD-DVR's at a fraction of the cost of the DTV model, and lower programming costs. Dish offers 15 HD channels currently, but to get the former VOOM original channels you have to buy a second dish and $100-install and pay another $5/mo. (total of $15/mo for 15 HD). OR you can pay $10/mo and get the usual 5 HD channels. DTV offers only 5 HD channels.
I can't recommend the HD-DVR enough, it's totally cool. Only Dish offers a HD-DVR on a lease ($250 one time fee). Only Dish offers HD receivers on a lease at no charge.
DireCTV has the advantage for quality of customer service, and has more stuff included for free, like off-air HD antenna which Dish charges extra for. (They are not difficult to install yourself). DirecTV has no plans to add any HD programming for at least 1-2 years. The new DTV satellite capacity will go entirely to local channels. HD locals, true, but no more.
Currently there are around 15 premium HD feeds that neither of them carry, and network HD feeds are only available in "unserved areas". The VOOM originals package that Dish picked up is impressive, especially for arts and sports and news. Whether it's worth an additional $160 in the 1st year is up to you.
Dish also offers all the Sirius radio channels, in addition to the 30 or so music channels.
Did I mention that the HD-DVR is totally cool? You can record 2 HD programs at once while you watch a different recorded program. You can record a HD program while watching another and pausing and rewinding it.