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Company I work for builds modular homes (not the same as manufactured/mobile)... We rarely use spray foam in any application. Usually build with 12" energy heel trusses and R-39 in the attic. If you don't have energy heels, just make sure the ends are as well insulated as you can get them.
You're talking about the roof, but you probably don't know the exact condition of your walls either. I'd highly recommend first getting a blower door test done to see where you are leaking.
I'd use the test results and focus on a better envelope by getting proper exterior air tightness/sealing.
Once you know you're airtight, then you can focus your efforts. You may find you need to add rigid insulation to the exterior walls as well.
If you are looking to have a well insulated home, you should install a good air exchange system with a heat recovery ventilator, etc because not only will it improve the air quality, it will help with heating and cooling costs.
If you do insist on spray foam, it can be better provided those steps are performed above. I say "can be," because spray foam installers don't always fill every crevice either and an air gap is an air gap. You have to be well-sealed one way or the other.
You'll probably spend 3-4 times as much on the insulation, but ROI in modern applications is much better at around 5 years. If there were any place I'd absolutely consider spray foam personally, it is in the attic.
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