hi, I asked the very same question last year, and did loads of research on the subject, even went to halfords so they could try different types of carrier, roof, tail gate and tow bar,
the up shot of it all was I did not bother in the end, the roof mounted ones seem to be better but you have to buy the 3 components (if you dont have any allready) ie bars, cross braces and brackets, but you have to lift the bike onto the roof, and strap it down.
no mean feat on your own and the wind noise is horrendous,
the tailgate and tow bar carriers were easier to use, but have problems.
1. when the bike/bikes are mounted it reduces the turning circle you have when the van is attached (this is suprisingly small on left hand turns because of the jockey wheel)
2. you cannot open the tail gate, when bikes are mounted so no access to the boot unless you unhitch and remove the bikes/carrier
3, the weight of the tow bar mounted ones plus bikes have to be taken off the nosweight allowance for the van, (this may not be an issue) but in my case it was as the Meriva only has a nose load of 50kg .
I even looked at the carriers that mount on the back of the van (like the ones fitted to m/homes) but these were not recommended for use on vans as the rear panel is not strong enough,
I did take the bike but carried it in the van, and put the awning poles in the car (same weight) a better solution and a lot cheaper. IMHO.
hope this helps.