A fixed are will work best if your sheep are in the same field all the time. If you need to be mobile, hurdles are the thing. Our sheep don't have a shelter as they move round the paddocks here and go out to temporary grazing through the season.
We bought ours from McCaskies in Stirling, way back in 2007. We have a mix of 6ft and 4ft. A 6ft and a 4ft in the corner of the barn makes a perfect lambing pen. Our hurdles have weld mesh over the bottom 18" maybe, so that lambs can't get through.
The ones we've got at our rented grazing are much heavier than our own ones plus they are just a couple of inces higher which makes stepping over them just that wee bit more precarious.Two points worth considering, I think.
Also look at how they join together - if you are using one as a gate, you want to be able to close and secure it easily.
Also remember, different makes will probably not link to each other which is a bloody faff.
Just a few of my thoughts.