Personally, I decided against LendingClub for my IRA because it just hasn't been around long enough. I didn't want to risk my retirement on a new type of investment (new to us normal people anyhow).
Also, I'm now doing a Traditional IRA instead of Roth, to reduce taxes. I don't want that money in LendingClub because it would be hard to do Traditional to Roth conversions later on. I'd either have to depend on the secondary market to sell notes, or let my cash sit idle long enough to build up enough to roll over. With Vanguard, I can simply have them transfer X shares of an ETF to a Roth IRA, without actually selling anything.
My vote is for Vanguard.