I use both and I think they are both as good as each other, they both detect, which is what i want, the Mk3m seems more watertight but i dont ferret in rivers, the only draw back with it is, as someone already said, you can forget to turn it on, i guess the only drawback i have found with the Mk3 is you have to remove the batteries after use, but if someone is too lazy to spend 10 seconds removing the batteries, then surely they won't want to walk miles ferreting.
If you need another collar, get the Mk3m, but i wouldn't bother "upgrading" to one.