We used to have mag signs as we undertook several Sub-Contract jobs that we had to go in "invisible" and as per the contract holder, so the ability to have a plain van had its benefit's. I also used mag's as I used my own 4 x 4 for work which mean't weekdays it was a company car and at weekends it became a personal vehicle.
However, I did find we lost several signs, either with the wind and rian or by forgetting to take them off and having them stolen??
We now opt for sign written vans, the main reason was the amount of vinyl we could have against the cost of a mag strip! The contracts we had were not jumping for joy about our signs but realised we are contracted and couldn't nor wouldn't take work form them. Thats not professional and we don't bite the hand that feeds us!!
The upside to our sign written vans is we have "honestly" generated work from them. Its funny we get calls saying we see ya van and blah, blah, ..... I have also had people calling to tell me how irratic my drivers are, oops!
Either option works, I guess it just depends on what you want from your vehicle.