There are several ongoing threads on which plan has the best coverage and how much data you use, etc. You'll find ten different answers every year as plans, companies, and coverages change; and all depend on the perspectives of the users. I'm jealously holding onto a Verizon legacy plan with unlimited data, so that's my perspective.
Here are some more general observations based on last year's experience and reading others' experience that are as unbiased as possible....
It's quite possible to exceed 5Gb but harder to exceed 10Gb in a month of active chasing, with 300-500Mb on a heavy day just letting apps run. It's also possible to get what you need with a lot less.
Ironically, what helps keeps the data use in bounds is high speed EVDO coverage -- or rather the lack thereof. Once you leave the large metropolitan areas and major interstate corridors the coverage is 1X, which is fast enough to get data updates, but not so fast you're tempted to watch streaming video, etc.
On the subject of coverage, IMO you want to identify the "no coverage/roaming" areas with each plan from the carriers' own sites and select based on that rather than their high-speed coverage. For example, Verizon has a shrinking-but-still-frustrating no-coverage hole in northwest OK; coverage in some west Texas areas such as northwest of San Angelo is non-existent with any of the major plans.
Live radar isn't really that big an issue since the images aren't huge and only update ten times or so an hour. High-res satellite can bust the bandwidth, however. The images are large and one is tempted to click frequently for an update, which most often results in a refresh of the same loop.
If possible start your plan two weeks before your chasing. That effectively doubles the data limit for the first month of chasing. Get your hi-res satellite loops and planning data from hotspots or wired locations and avoid streaming a/v stuff if data use is a problem.
From my experience the SPC mesoscale analysis pages provide a full menu of readable data and images updated to current with a click that are easy on bandwidth. Only fire up the big guns on the road after chaseable storms initiate