I'm going to be relying on a pair of Garmins for riding and recording - an edge 520 and an edge 820 explore, which are both small and light. They run for about 8 hours on the internal battery and take very little power to recharge so you can top them up a lot of times from a battery pack.
I paid for my 820 for £230 from cycle republic thanks to careful internet shopping. Keep an eye on prices and vouchers. I believe the 520 is currently well under £200.
Both units are very similar, except:
- The 520 has about 100mb of free space, and the 820 has 16Gb of space. So the 520 can fit all the GPX tracks, and all the logs, for LEL but the maps lack detail. The 820 can fit high-detail maps of the whole of the UK (and most of the rest of the world).
- My 820 is the cheap version ("explore") so has no power meter support or WiFi.
- The 820 can do turn-by-turn navigation with any route file, not just specially formatted TCX ones. I've turned this off, though, because with it enabled it takes ages to load a GPX file as it has to process the file to detect the turns.
In both cases the units work very well for basic navigation following a GPX file. I'll have the 820 set up to show a map and the 520 set up to show elevation profile and stats. The screens are easy to read during the day, but a head-torch is required to read them at night.
Touch wood, I haven't had a problem with either of them crashing and losing data, and they both handle complex 200km GPX files with few issues. But if you really want to make sure that you have a record of your ride I don't think anyone would recommend relying on a single device.
If you are going down the mobile phone route then an Android phone and OSMAnd is highly recommended. It has full offline maps, the search/routing works well, and you can read it in the dark. You will need a very big power bank to keep it going for 4 days though.