FWIW, I had narrowed my choices down to the F2, and ACE 250. My research indicated that both were very compareable in performance and features. I purchased from Kellyco, and, my final decision was based on the warranty. The F2 offers a 5 year warranty, while the ACE 250 only offered two years. As I stated in my earlier reply, we bought my son a ACE 150. That purchase, I don't regret either. It's built very well, and, comes with excellent documentation and instructions. At this point, I recommend either company.
What you need to do, is decide what you want the detector to do for you, and what features you want. Go with the model/brand, that fits your requirements.