One word- certification. If you don't have experience with antenna performance and connectivity, it's worthwhile to get help either from a reputable consultant or from the module manufacturer. I am starting to see a few M2M antenna design consultants out there, but I haven't yet worked with a customer that has used them. If you struggle with your antenna design and performance, it will delay your launch and could cost you a lot of money if you have to go through PTCRB/FCC certification more than once.
Also, keep in mind that many carriers have pre-certified modules, and if you don't select a module from their pre-certified list, it can delay your launch and cost you a lot of money.
It might seem easy to attach an antenna to a CE device and have it connect, but more than half of the customers that I work with struggle with this, don't pass certification, and never launch their product.