My advice is always honesty is the best policy - if you prefer full time permanent let the client know that you will take her one but should a full time client come along and want to commit to the space she occupies her option will be to start to pay full time for her wonky spot she is occupying OR forfeit the spot?

This way later you are not dealing with guilt or the concern of bad karma for having taken her business when it suited you and than turfing her for something better ...it is not a personal thing just business in that you are saying I can help you out now however this is my long term business goal and it could take me a month or 6 months to fill that space, someone else could leave in meantime and so forth so you could end up being able to help longer term than expected?