Why buy it once as mechanical when you know you're going to refit with a hydraulic thumb kit a year or two from now? Get the factory installed hydraulic thumb right from the start and avoid the hassles of plumbing and making it fit later at a higher cost than the present. Two years ago I bought a new Woods BH90X with an 18" bucket AND a hydraulic thumb for $7000.00-Rosseel's. Now it is $8050.00 per ebay. Sometimes it makes good sense to "get on the bandwagon" from the get go.
MIE is a good company, if they offer a hydraulic kit for their thumb, don't hesitate, buy it and be done.
I have two of their custom in-house products, both of which are stellar in their designs, a loader bucket grapple and a BH ripper tooth.