Hey Middy, welcome to the fold.
Im jist down the road from you in Cleethorpes 👍 quite often riding through yiur neck of the woods to play on the wolds or head up to cadwell. Also frequent Willingham woods 1 or 2 Wednesdays a month in the summer.
As for your bike dilema, I personally would take the older fzs600 with the low mileage. They are still cracking bikes and if it's been looked after it will last you a lifetime.
With so few miles it should still be like a new bike 👌