If you have an old no-longer used phone, and if the available phone battery life is okay for your usage, and if you either don't ride in the rain or your old phone is reasonably waterproof, then that's a decent option. Keep your current phone safe and sound in a pocket, and let the old one hang out in the elements exposed to risk of damage/theft. RWGPS and/or Strava, occasional wifi connections, and you're good.
If that doesn't work out, for just speed and distance, I think it's a tossup between a low end GPS and an old-school cyclocomputer, with each having its strengths.
If you're not going to upload ride data or download routes, then the battery life difference between the two might be the deciding factor because it's, uh, hours with GPS vs months with cyclocomputer.
The cyclocomputer is fiddly to install, and has to be configured for wheel size.
The GPS is more likely to be stolen, will lose data in heavy cloud cover/tree cover/tunnnels, and they do occasionally crash.