Get Summon Arvak if I choose the Dawnguard side

The short answer is No, the balance of the game won't be noticeably affected.

Dawnguard introduces a few new things that modify minor aspects of the base game:

• Dragonbone Weapons: With a smithing skill of 100, you can now craft weapons from Dragonbone. The bow has the highest base damage of any bow.
• Arrow Smithing: Able to smith arrows

• Crossbrows: Faster first shot with slower reload time. You can smith an Enhanced Version of this cross bow that ignores 50% of armor. This is pretty huge and all you need is the steel smithing perk.

• The vampire and werewolf perk trees are also available, and improve the base abilities for those factions. These are gained by feasting on the flesh of your fallen foes (aka devouring a killed NPC as a werewolf or killing an NPC with drain life or bite as a vampire lord)

So what you're truly given with the DLC is some interesting build depth for players who enjoy playing as a Vampire or Werewolf, slightly more powerful weapons for those who reach 100 smithing, and a bow substitute that can eventually be improved to ignore 50% armor at high levels, but needs to be earned through Dawnguard questing.

Looking at all the info I think you can reach an objective conclusion that current players shouldn't notice any major balance upsets.

Everything that affects bows affect crossbows as well (damage multiplier etc), even Deadly Aim. Those perks that aren't immediately obvious work like this:

• Hunter's Discipline - more bolts (from mobs who drop bolts in the first place)
• Ranger - move faster while aiming and reloading
• Quick Draw - faster reload

Update: The mechanic of the weapons can be confusing.

• Bow: Ready-draw-aim-shoot-ready.
• Crossbow: Ready-aim-shoot-reload-ready.

Quick draw yields faster draw for bow and faster reload for crossbow. Ranger lets you move faster while aiming with your bow or crossbow. The thing with a crossbow is that you aim an already loaded weapon and with the bow the draw action is part of the aiming. In the end, it's no real difference.