We've got both, use Ventura when touring on the 'nice' bikes, and Givi top box when using the commuters. We have found the Givi makes a difference to handling (CB500!), but it is very convenient, and you can leave two helmets securely on the bike (well, when I say securely, it would be very easy for someone to force a topbox open if they wished to do so!).
Depends on if you want to have the luggage on all the time, or only when you are touring. If all the time I'd go for Givi, if only when touring I'd go for Ventura (you can remove the rails between trips and the bike returns to its former look).
Its no good looking for the needle if you have already lost the thread.