My phone is waterproof, glove friendly (at least with my gloves), never overheats, and was cheaper than either of the GPS units mentioned in the OP. Data plan is something I have already, so no further costs on that, and as I stated - I build my routes on the computer and don't even have to move them to the phone, they just show up on the My Routes menu. The TomTomGo app is also offline, meaning that just like a regular GPS device, all the maps are stored on the device so it works just fine whether or not there is cell service.
I don't think it's overstating things at all to say the dedicated GPS offers nothing my phone does not. I only have 1 SS1000 thus far (done using my phone, as it also uploads to SpotWalla, which the GPS cannot do) but plan more, and have done many non-IBA rides with my phone as my guidance. I'll be happy to see if I run into any situations where I want a GPS mounted on the bike. So far, though, I have not.
Whatever you're comfortable using is the big thing, but don't downplay using the phone. It's a far more valuable tool than some may think it is.