A lot of rail systems are designed to be lightweight, for example ones by Noveske, Troy, Daniel Defense (specifically the MFR) and PWS. All of said manufacturers have some of their designs replicated by Madbull.
They're high quality and licenced appropriately if you like trademarks, but expensive as a result.
Next there's the length, you may be limited to specific models. For example, most are available in 7", 9" and 12" lengths; carbine, mid-length, rifle. But there are certain designs which are available in / only made in 15" or 4" etc.
Admittedly that probably isn't much of a problem but it can be one thing to consider dependent on what your gun is like and narrows down the options.
Relatively new on the market although seemingly really popular (as more companies, real-steel and replica makers, are producing them) are Keymod systems.
Again these are designed to save weight however most of the companies will sell the rail panels extra and these can be very expensive. It can increase your choices of attachments as there are parts specifically for attaching to Keymod rails (mainly lights & light mounts, foregrips, QD sling points)
I'd say it's really just preference. Main brands you'll come across, as far as I know, is either Madbull or ACM (Chinese clones) With the former you don't have to worry about quality, it will be good.
With the latter it depends but generally quality will probably be alright and paying £30 might be worth it more than £100+
