Yes, the gamestyle and rules are completely different - there has been some crossover with graviton weapons (you'll be familiar with the 'graviton imploder') but there are no doctrina imperatives or canticles of the omnissiah.
Models with a cybernetica cortex need either the 'paragon of metal' upgrayedd, or an
HQ choice with a cortex controller within a certain distance to keep it functioning as you want it to. (it may just go for whatever enemy inside 12" which is closest - even if the assault target is 12" away. this can be funny with outflanking vorax.)
Mechanicum forces have a dedicated transport - the triaros which is a rather good transport for the points
The downside of the mechanicum is that points costs can rack up pretty quick; but there are ways around that, like using thralls as troop choices for objective capping.
The main difference for me is that all the unit choices are really cool and have some nice tricks and playstyles, but also have an obvious drawback - unlike the 'adeptus mechanicus', there are no free buffs.
It's a bit like a cursed wishing-well; you get your wish but theres a catch....