I had a health equity account through a previous employer. They required a minimum balance be maintained in cash, I believe it was around $2000. The rest could be invested. The funds I had access to were alright, but not as good as something I could have found on my own. I don't remember specifically what I invested in, but it was an index fund with the lowest ER.
I opened my own account at fidelity which has no required minimum in cash (ie 100% of the account can actually be invested). It also has much better fund options, including and index fund with an ER of 0. Yes it actually has an ER of zero, I don't know how they can offer it, I guess it's a loss leader and they are hoping you'll bring more accounts and money to them.
If this is through your employer I recommend you use whatever company they have set up. You will not have to pay FICA taxes if the deduction is done through the payroll of your employer. If you opt to open an account and fund it on your own (ie not via payroll deduction) you will still get to deduct the income, but you will have to pay FICA taxes. You can still open your own account at fidelity though, and move the funds over periodically. That way you can take advantage of not paying FICA by going through payroll, and then still have your choice of what company you move the money to after that.
If this is not through your employer and you are just opening one on your own, then I definitely recommend fidelity.