Honestly, $5 for an extra 3GB is a no-brainer to me but if you're finding yourself never even getting close to 5GB, then it's worth going with the cheaper plan.
In my household, while we don't travel all the time, when we do, everyone tethers off me. Those days, we chew up a lot of data, so the extra 3GB is worth it just for my peace of mind.
I don't think the cheaper plan is "less likely" to get a price increase. I figure they'd all be subject to that possibly.
Public Mobile $38/5GB