This is generally just based on the user's preference, but I would say you would be better off using sign in, but be consistent.
You see, 'log in' does not generally mean enter the system with credentials, logically, while 'sign in' is more recognizable. Take for example you are going into a hotel, and you need to get a room. You use your credentials (Your ID, Credit/Debit card, etc) to get the room, and when you get to the front desk, they sign those credentials into the hotel's database/log. I may be the only one, but log in is generally more confusing, and does not make as much sense as sign in.
Then there comes consistency; you don't want to use something that is not widely used, as it will be overall more confusing and unacceptable compared to the majority of services. Take for example this chart:
http://www.designcult.org/2011/08/why-do-we-call-in-logging-in.html ~ chart source
Quite easily, you notice that 'Sign In' is very much used, as opposed to log in, login, logon, or another word/phrase. This is generally leaning you in the direction to use Sign In because it will be something that people will recognize faster, and it just makes more sense generally and logically.
It is up to you in the end, but I would generally say that Sign In is the better option, given the fact that it has already found it's place in the computing world as the main way of saying 'enter the site/service here!', and it is more logical if you think about it.
I hope this helps, as it is more on the confusing part of UX design.