Mauro,
Where will you be hunting...salt water, under water, wet sand, etc. ? Remember that the Infinium is a gold hunting machine first, and has to be tuned for salt water use and has to be ground balanced. Saying that, once set up to run stable and smooth, it's a awesome beach unit with the 10x14 mono on it. It goes deep and hits low conductive targets hard. The secret to that machine is to make sure it's tuned to the conditions and if you can't get it just right, you have to hunt with some chatter and repeat sweep over the target to make sure it gets a repeatable hi/lo or lo/hi signal to distinguish it from a false signal.
The Mark is a much easier unit to use and does not need set up. I never used one, but it seems similar to other beach PI's in that there isn't much to do to start hunting.
In my opinion, there are major differences in the two units, and you should read up on them to make a determination as to which one may be best for your hunting applications.
If I had to pick...Infinium all the way, but be prepared for a learning curve and you have to have patience with that machine.
JC