Just pointing out alternatives that aren't the Black & Decker variety. Do what makes your life easier.
I just got tired of mixing fuel, tune up's, running out of gas, fouled plugs, tough starts, spilling fuel, winter storage, loud noise, leaky tanks, hot motors, ect. and it appears that even the traditional powers see a market for cordless.
Gas doesn't beat battery in yard work in my life, not close. Now, I live in a subdivision in the city, not my father in laws farm, so that does factor into the equation. If you're trimming drainage ditches on county road 100, well then I'd probably put up with gas too. However, it takes me about 15 seconds to get trimming each and every time for the last 3 years. I too hate running out of battery in the middle, so I bought an extra battery and a fast charger. I keep the extra battery in my pocket. Problem solved. I wouldn't take a gas trimmer of any kind, even if it was free.
All that, and I am interested in your decision as my FIL will probably need a new gas trimmer in the near future.