Check a few places in addition to Sofi. Earnest (already mentioned) is one, my DH and I refinanced with them and are really happy with them. However, one downside is they recently did some security changes and no longer sync with Mint, if you use that... so its like they block Mint and they have no plans to change that. So keep that in mind, because the loan will not update in mint as you pay it down. But if they end up giving you the best rate, you should take it, because that is optimal. I also have a referral if you are interested.
Also, if you anticipate needing or using the perks that go along with federal loans (income based repayment, deferment, etc), I would hesitate to refinance at all. DH and I refinanced $150k in loans, but that was because we have stable, high paying jobs and didn't need to defer or use IBR. Losing those perks when you need them can very quickly overwhelm you financially.
I've also heard good things about Lendkey and Credible. Aim to check out 3-4 refinance places and then pick the one with the best rate/terms. Shop around. Some will offer lower rates for higher monthly payments. Up to you if you want/can swing higher payments. Also, you don't have to consolidate both, you can refinance only one of your loans if you want. One way may yield better results (ie just refinancing the lower amount loan may offer better rates than asking for both combined).
What is your payoff timeline on these? Refinancing may only save you a month or so of payments if its a short timeline (under 2-3 years, give or take?) Especially on that $6k loan, that is relatively small plus your highest interest. I'd throw as much extra at that loan as possible and just pay it off... saving 1% on it may save you $100 or something total. I'm totally spitballing here, but you should carefully measure how much interest you currently pay and how much you would pay if you refinanced.