We noticed a unique occurrence with our son who is about to turn 2. About 5 months ago while in the crib we noticed that he was not sleeping well and waking up a lot during the night. He would also wake really early and when he did, he was mad and crying. Then he started telling us, "DOWN, DOWN!" Desperate and sleep-deprived, we took a chance and put the crib mattress on the floor and started telling him "big boy bed". Not only does he love it, wants to show everyone and suddenly has seem to mature practically over night, but he also sleeps better. I think the freedom of being able to come get us if he's afraid or needs comfort helps him to sleep better. And when he does come to get us, we don't make a big fuss, we just keep quiet and tuck him back in. For those tough moments when he doesn't want to go back to bed, we again bring up the "big boy bed" and he needs to go to sleep because it's night night time. It has been much easier than what we were going through with the crib, and him being very unhappy about being trapped.
So having said all of that, I think the spring or foam decision is completely yours. We've had both and had accidents on both and neither are fun or easier to clean. And they will be happy jumping on either one.
But tips for transferring to a big kid bed: make a it a big deal for them, maybe even having a big kid bed party with grandma and grandpa etc and getting new sheets or a pillow that they like. Explain in kid terms that at night time you stay in your big kid bed just like mommy and daddy stay in their big bed. Praise them over and over when they get it and get excited about sleeping at night time. If you're desperate and nothing else is working, tell them that they have to sleep in their crib again if they can't stay in the big bed. Oh, and also be aware that some kids find comfort and actually WANT their crib. A friend of mine has a daughter who turns 3 soon and they just made the transition!
Hope this helps!!!