What features you NEED, well you really don't NEED any, a good map and a watch will give you rough data. What you want, that's a different matter, I would say the most important feature is trip distance, followed by elapsed time, then saddle time, then speed, then cadence, then odometer distance. Realize that wired units are more reliable the wireless, and if you use a trainer in the winter, then a longer cable can be handy. Don't count on instruction book distances being accurate, you need a helper to do it right, first pump tires to normal pressure, then with the valve at the bottom, have your helper mark a line, then ride 5 wheel rotations, and mark another line, measure the distance between the two marks divide by 5 and convert to millimetres.