I've never used Betterment, so I'm not sure if they have any account closure or transfer out fees, but generally speaking, I would say there's no reason to wait. The main "cost" would typically be taxes, but since you have a lost that's a non-issue.
Typically, if I'm moving investments from one brokerage to another, I will do a "transfer in kind" which means I don't actually sell the investments, I just move them from one place to another. The advantages of this are twofold. First, it's not a taxable event. Second, if my transfer gets stuck in limbo, I'm not running a risk of the price changing dramatically while I'm out of the market.
I don't know how Betterment investments are structured, so I'm not sure if in kind transfers are possible. If you're worried about prices changing mid-transfer, though, you could explore this.