Disco 2 16 or 18 inch rims?

My D2 has 18 inch rims with Grabber AT tyres, cant remember the actual size I think its 255/55/18?
anyhow, I was wondering if 16 inch are better option, I notice so may have 16 inch rims, I guess this was the standard size, I know the tyres I have are summer type so want to get some chunkier ones for playing with off road and was wondering, should I get some 16 inch rims and tyres to fit or should I stick to 18 inch rims?

what have you got and what do you think?

I have both 16 and 18 inch rims. Either size was supplied by LR, depending on model of Disco 2.
I use the 18s during the warmer months with 255/55 R18 tyres and the 16s have cold climate 235/70 R16 tyres.
To all intents and purposes, the overall diameter is similar, especially if you stick to the Land Rover recommendations in the Owner's Handbook. The tyre sizes I've shown there are the recommended ones for the alloy rims. The 18s are 8J and the 16s which I run are 7J. There is a 16 inch 8J rim size quoted too and the recommended tyre would be 255/65 R16.
The difference between them is that the sidewall is actually deeper on the 16 inch tyres so there's more compliance, even at the recommended pressures.This tends to give a softer slightly more bouncy ride whereas the 18s do tend to give a slightly harsher ride.
If you are going to provide yourself with an additional set of wheels, remember that the Disco 1 and Disco 2 have a slightly different stud pattern.
Unless you're going full time, all out "mud plugging" and almost never using the Disco as a road car then a careful choice of multi-terrain type tyre might probably be a better route to go.

Raod driving 18", off roading 16"

Mine are standard 18" with 4 season tyres. I use it 95% on road. They're OK off road but not great. The plan is to retire the Disco from family duties in the summer and turn it into a weekend toy. It will need different tyres then almost certainly on 16" rims.