The first question (as always with this type of topic) to ask ... "What you you use your vehicle for?"
Is it a daily driver, do you do frequent towing and/or shift heavy loads ?
If you do, parabolic springs may well work for you.
For normal Land Rover type use, standard springs and shocks will work just fine.
It's like comparing apples and pears.
60 year old rusted up springs vs brand new (quality) OE spec springs.
Some years ago all the glossy Landie mags raved on about parabolic springs and how they gave a feather bed ride.
Tosh and nonsense, they won't. There are several British spring makers now producing decent and long lasting OE springs (and parabolics).
You'll need to price up and see, but the cheapest won't necessarily be the best.
I've not bought new springs for some time so can't help.
I did buy a set of gas shocks once - £40 each (Pro Comp IIRC) and they were rubbish.
Can't beat decent OE shocks, but try 1-ton shocks all round if you have a SWB, they seem to work better, for me.
Parabolic springs will raise the vehicle height by about 2". This makes it easier to roll under and do repair work without using a jack or axle stands - BUT as posted, might mean it won't fit in your garage (or under council barriers at the tip).
You also have to consider the length of the flexi brake hoses.
And if the rear prop shaft is old (and worn) the change in angle can be enough for the shaft to grumble.
As ever - it might help you make your mind up if you can beg a ride/drive on one already fitted with parabolic springs.