It's great to say that markers never fail, but sometimes they do, and it isnt always predictable. Could even get so messed up with paint in the breech causing probs that it is just easier to pull out the trusty backup and use that for the rest of the day.
Ok, and we all say that I use good paint and it never happens, once I got shot in my speedfeed which caused a hell of a mess which the hopper instantly pushed into my breech cos all the paint just went splat!
You want something that is reliable, and unfortunately an Ion isnt it. Deffo go for a Proto marker, as you will be familiar with it.
I run a PM8 with a PMR as a backup. As long as I have lubed the PMR bolt in advance of a gaming day on the rare instance where I have had to use it, it has never let me down.