I have tested three different aerials, directional and two omni directional, one omni being the high powered.
The difference between the high powered and the cheaper omni directional worked out about 8% better, so I always recommend the cheaper one that we have in the
Motorhome Craic shop @ £25 (cheaper than the main supplier inc postage)
The directional aerial came out around the same as the high powered, about an 8% benefit.
I have used the cheaper aerial for well over a year now with no problems.
In saying all that, I have three systems being tested in France at the moment by Mark and will have even better results once he returns in a week or so.