Hey someguy. You can definitely use the Guthook Apps without a data plan-- all it really requires is a GPS signal, which doesn't require cell service, and to download the maps you could just use a wifi signal when you're in town. I would recommend using the iPhone sparingly, since the eTrex's batteries would last longer and be easily replaceable in the field, but if all you need is a spot check and a quick satellite fix, you could get several days of use out of the iPhone while leaving it in airplane mode most of the time. Just make sure to charge it when you get to town.
I'm not familiar with SPOT's iPhone app, so it might be worth sending their tech support people an email with the question.
Good luck on the trail!