Having a partner is a much more involved, permanent type of fix. An employee can be replaced with not too much effort (the good ones are harder to replace, but still replaceable). A Partner, it becomes more of a legal issue, much more involved in dissolving. If the partner doesn't work out, you could lose all you've worked for. I have a partner, my mom, but had worked together before so we knew we worked well together. We started together from nothing and built it - together. You've done the hardest part - the start up and getting the initial clients. I think it may be a mistake to now hand part of it over to someone else. It's like making a delicious caramel apple pie with vanilla ice cream, whipped cream and cinnamon and handing it to some else to eat! Sorry, I was trying to find an example that might make sense!
Anyway, to me, it would make more sense to find an employee to relieve you of your overwhelmedness (if that's a word). But, you mentioned that you would have them do marketing and administrative tasks. Nobody will sell your business like you do. Marketing is usually the most disliked job in the office, but you may find you are wasting money having someone market for you as an employee. Unless they are a professional marketer and don't mind it. The things you should be doing as you grow are working on your business, not in it. You can either hire someone to work from their own home (not my recommendation) and pay them a salary, hourly rate, or some form of agreed upon amount, or hire someone to work in your office. Growing pains!
Best of luck!
Michele