I agree with others. First, you will absolutely LOVE the Solstice - it's a wonderful ship. And you really don't need to worry too much about seasickness on that type of ship, but book something towards the middle instead of the front to be on the safe side.
And as others have stated, if you like to use a balcony, then there's nothing like it. But if you're not in your cabin all that much, then an oceanview will do just fine - not much difference in size and really no difference in the cabin itself. And once you leave the cabin, everything else is equal.
For a first cruise, you've selected something that'll be very difficult to beat! You'll definitely be spoiled.
And as for booking with an American agent, we have several Australian clients and they've all stated the same thing you said; it's usually cheaper than booking with a local agent.