i feel your pain. i had an aging 3g on fido myself, luckily my hardware upgrade window opened right when the 4s came out.
but it is a tough call. telus has a really attractive $70 plan that gets you 6gb/200 min/my10 that part of me wishes i'd hopped on. i called fido retentions to ask them if they could do anything comparable to what others offered, and they knocked about $10 off my monthly bill... but i'm still stuck with 500mb data and having to pay for a canadian long distance package to keep in touch with parents, when the my10 option with other carriers would have rendered it unnecessary.
all said i'm paying an average of $5/mo more than i would with a different carrier, but that works out to $180 over 3 years, which is less than the $200 ECF and $100 in fido dollars that helped cover the 4S hardware cost. there would have been less 'baggage' if i was closer to the end of my contract, but as you said, the 3g gets pretty frustrating, and a person's time and annoyance are things that need to be weighed as well.
you really just have to weigh out your personal needs and decide which option is best. good luck.