Your question is a fundamental one that has been debated extensively in academic literature. It is the "make or take" question. You have already identified two major factors in the decision process, the time to execution and the probability of execution. The probability of limit order execution is dependent on the distance of the limit order price and the inside market, the position of the order in the order queue, the stochastic order cancellation rate, the stochastic arrival and size of liquidity-taking orders, and other lesser factors. If someone were to answer your question with some percentage, their answer would be highly subjective and dependent on randomness.
How exactly limit orders are queued and executed is very specific to the exchange in question. Some electronic order books use a price/time matching algorithm. Some also have an auction algorithm working in tandem. Market orders are always given priority over limit orders in the markets I am familiar with.
You might consider a combined approach. The fact that you are considering a limit order strategy at all implies that you have the luxury of time. You could submit a series of limit orders for a specified maximum time limit, then cancel and resubmit market orders for the unfilled remainders. However, I think you will be better off selecting one strategy or the other, and avoiding this kind of complexity.
Is the difference between the bid-ask spread really the difference between profit and loss for your strategy? If it is, how sure are you that you are not trading white noise?
-segv