I plan to upgrade my FR 945, mostly due to its disappointing GPS accuracy. I use the watch for running only, so both FR 255s and 955 should do the trick.
I like 255s due to its smaller form factor but 955 has map and longer battery. Admittedly, I used map only a couple of times a year but it seems handy when needed.
Another possibility is to use 255s to replace my aging Vivosport, which 955 cannot do. However, I am not sure if I can sleep well wearing a watch. If Vivosport is dead, I may still need to buy Vivosmart, regardless 255s or 955 I will own.
255s is $100 cheaper than 955 but that is not a major factor.
Please let me how you think.
