The practical failures, which are more likely to affect you than the beam pattern.
Battery life is quoted at the lowest setting. A light claiming a long runtime often gives you a fraction of that on the setting you actually use. Check the figure for full output, and assume less in the cold, because batteries lose capacity in winter and that is exactly when you need them.
Charge on a schedule rather than when it gets dim, since most of these fade gradually and you will not notice until it matters.
Carry a small backup, front and rear. It does not need to be good - it needs to exist, so that a dead light is an inconvenience rather than riding home unlit. This is the single most useful thing in this thread and it costs very little.
Check the mount. Cheap rubber mounts perish and the light ends up pointing at the ground or in a hedge. This causes more failures than the electronics do.
Two rear lights is worth it if you ride a lot in the dark: one on the bike, one on the bag or helmet at a different height. Redundancy for the direction you cannot see.
And for your existing pair: keep both. The small one is your backup now, which is a job it can do well.