The distinction between the two methods of distribution becomes less important over time though.
Assuming in a year or two both systems have distributed 50%+ of total posible units (already true of Bitcoin), how will they differ afterwards ?
Bitcoin will continue small incremental distributions effectively indefinitely while Ripple will have that 50% "Fort Knox" stash in the background.
Do you really not see the difference between
1. 50% of the coins are in circulation, the other 50% will be distributed according to a fair, objective protocol; and
2. 50% of the coins are in circulation, the other 50% are held by a central authority to do with as they please?
Even with the unwarranted assumption that OpenCoin is noble, what about the security implications of a single party storing an amount of currency equal to the total in circulation by a single party that can be hacked?
I agree that the initial distribution doesn't matter
that much, but it needs to be reasonable. If it was possible, distributing bitcoins equally among all people would be superior to PoW, but that's impossible so PoW is good enough. But centralized distribution is not good enough.
At that point various scalability issues in Bitcoin (TPS ceiling, blockchain size, etc) will be contrasted against scalability strengths of Ripple
What scalability strengths? Maybe I'm missing something but AFAIK each transaction in Ripple requires much more processing than in Bitcoin, hence it's less scalable. The ways to increase Bitcoin's scalability are already known - I'm sure such ways are possible for Ripple too but they're not as widely studied.