I find the Petzl Dart and Dartwin the best fitting crampons over all between offerings from the BD, Grivel and Petzl. The slim fit on the front bail is one reason. The Petzl heel levers are a little tight on my 45s but work well enough. The BD heel levers do work better how ever. So I use them. Easy on and off. You can buy the spare parts from BD at a small cost. You'll find it a much more positive latch up as well because of the slightly differeing leverage.
I've never had the sidelock versions fit any of my boots very well.
I haven't used the Sarken but would expect the same fit and heel lever issues from them since it is the same heel piece and toe bails used as the Dartwin.
My suggestion, is keep the standard front bail. Buy a set of BD heels and make sure the length adjustment is right on the connecting bar. I use a normal straight, long bar. The big rocker in the heel of the Spantik doesn't help when fitting the crampons tightly. That is the reason you want to be sure on crampon's sizing. As a last resort it is not unheard of to take a belt grinder to the sides of the heel on a Spantik either to make it fit the crampon heel piece better. It has been required on some of my previous boots but wasn't on my current pair.