This is a draft of my thoughts on GPS - feel free to comment/ correct and I will try to update it.
Many recorders use a basic Garmin etrex GPS. Traditionally this was yellow like the one below, with the globe symbol on it. This has now been replaced by the higher sensitivity model which does not have the globe symbol - it usually says HC or High Sensitivity above the screen. There are a few different models these days such as the etrex Legend, Summit or Vista, which have slightly higher functions and additional buttons.
|New-style Etrex H|
One of my main objections to the basic Etrex H is that there is no page displaying your grid reference (coordinates) live as you move around. If you want to know where you are I find this very useful. If you want to record a plant you have to go to the menu page and select "Mark" to create a waypoint. However, if you then walk 100 m and want to know again you must return to the menu and press "Mark" again - it will not update as you walk. I prefer the Satellite page on my Etrex Legend, which shows the grid reference changing as I move. I think this makes it easier if I want to write a grid reference down.
If you don't like writing down grid references in the field, you may be able to save waypoints which you can download to your computer. This could be useful if you are trying to record a lot of grid references (e.g. for a detailed survey of a population of plants). It is possible to download data from the basic Etrex H, but it is easier with the more advanced models, which come with a basic software package on a CD.
The basic Etrex H only has a serial connection to your computer, which these days is getting a bit harder to use (and you may have to buy the cable separately). The more advanced ones now come with a USB cable which is easily used with any modern computer.
One of the other differences is in the screens - mine bought in 2011 is a Legend with a colour screen - the older ones have black and white screens. Colour may use more battery power, but if you can function well with rechargeable batteries it may not be a problem - they all take standard AA batteries. Mine lasts for more than a full day in the field when it has freshly recharged batteries, so I'm not complaining.
The Etrex H starts advertised as £89.99, while the others are more expensive. However, if you search on Ebay or Amazon you may find significant reductions.
I have now seen a new Etrex series - Etrex 10, 20 and 30, which look very robust and should be waterproof. They are meant to be even more sensitive/ accurate as they can use the Russian GLONASS satellites as well as the USA GPS ones. The 20 and 30 are compatible with Ordnance Survey maps, and with the 20 priced around £140 this is I think the cheapest OS compatible GPS I have seen yet. I'd be interested to hear from anyone who has bought or used one of these new ones.
If you do have significantly more money to spend you might consider a model that includes mapping - you would have to buy OS 1:50,000 or OS 1:25,000 (more expensive) electronic map tiles. This is great to use as you can see exactly where you are on the map. This sort of handset should also come with software so that you could use the mapping at home on your computer (e.g. MemoryMap or similar) and plot your routes and waypoints.
If you are a smartphone user (or considering becoming one) there are several apps (some free, others with small to moderate cost) which will allow you to use maps such as MemoryMap and GPS. The GPS may not be quite as accurate as a specific unit, but should be pretty good. These do work (once you have the app installed) even when you have no network coverage, as long as you have satellite views.