What consequences will the choice of factions in Dawnguard have?

I haven't played the Dawnguard Questline yet, so please try to keep this free of any major story spoilers.

I'm wondering whether I should join the Dawnguard or the Vampires. Before I make my decision, I wanted to know if chosing either faction will prevent you from acquiring certain skills/spells/items/et c. For example:

• Can you still become a vampire if you join the Dawnguard? Even after you have completed the Dawnguard questline?
• Can you get crossbows even if you join the vampires, or can you get them only by joining the dawnguard?

On top of that, I'd like to know if there's anything else I might miss out on by choosing either faction. Thank you!

The Dawnguard won't hate you if you become a vampire after joining, though they will refuse to offer you services or quest progress until you cure yourself (and they indicate as such).

Regular Crossbows are available, even to those who side with the vampires. Bolts may be harder to come by, however. If you want more advanced crossbows you'll need to side with the Dawnguard.

Regarding other bonuses, joining the Volkihar Vampires will eventually earn you some unique vampire-boosting equipment, whereas the Dawnguard will provide you with unique crossbows.

The biggest difference are the master-level trainers -- Archery, Heavy Armor, Restoration, and Smithing for the Dawnguard, and Two-handed and Speech for the Volkihar clan.