It's only my opinion, but one gigabyte really isn't all that much anymore, not with Smart Phones and all the stuff they typically do in the background. A big determining factor too is going to be in how good the service coverage is for you in the areas you visit the most with the Phone, so how much 4G/LTE data you'll actually be using. I'm not familiar with the offerings from Comcast and how they work, especially for use with a Mobile device, but if it were me, I'd say around two to three gigabytes is a fair number for data while you're out and about.

It might very well be possible to use less than this, but the main thing to keep in mind is you'd want to try and Limit the number of services and such that are active on the Phone while you're using 4G/LTE. Basically this means that you'd want to limit the applications that used larger amounts of data access to the Cellular's data and try to use them when you were connected to your WiFi at home. either way, if you've got the green for it, I'd say that 45 bucks for unlimited data sounds pretty good, especially if the service from Comcast were covering both your internet from home and your Mobile data while you were out, that's pretty neat.
Hope this helps.
Take care.