Need to be more specific. Not all i5's and i7's are made equally. There are speicifc models that yield major differences in performance.
Generally speaking, the i7 models are hyperthreaded and the i5's are not. (I don't know any i7's that are not hyperthreaded, but I could be wrong since I don't know everything about everything). What this means is... each has 4 physical cores (or 6 for a few of the i7 models like the 3930K or the 980X).. however, the computer can read each of those cores as 2 threads instead of 1 thread. So... you essentially get 8 threads instead of 4 threads with the i7's. For DAWs that can utilize this properly, it's a very significant improvement in performance. Higher track counts... lower latency.. better stability... more VST instruments and effects.
So.. although you need to be more specific, it's safe to say that an i7 will beat an i5 for audio work, provided they are at the same clock speeds, mostly because of hyperthreading. Check the specs of the computer to see which exact models you want to compare... and then go to cpubenchmark.net and search for each in the "High End CPU" category to get a general overview of where they stand compared to each other.
What you may be getting caught up in is Gamer talk. Gamers do not need multi-threads, b/c most of their games will max out on 4 cores anyway. So... they could really care less about the hyperthreading that comes with i7's. It would be a waste of money, and the power in the i5 probably can't be beat in terms of price for core performance, if your software doesn't recognize multithreads. However... most audio DAWs do utilize it. But, you'd of course want to double check if yours does. I don't use FL Studio so I have no idea.
And lastly.. like krushing mentioned.. depending on your producing needs, you may not need all that power either way. I did just fine on an AMD Phenom II for the last 2-3 years, and those don't touch an i5 OR an i7.