I’ve never dealt with GHQ units, so I can’t comment on them.
I’ve spent a lot of $$ on HBG and OOB units and I can comment on them.
On detail, HBG units are more detailed than OOB units, especially on tanks and ships… HBG planes roughly the same level of detail as OOB (not significantly more detailed).
On scale, HBG units are well scaled to themselves… vs OOB units, there are some hit and miss or huge differences depending on which units you speak of.
On OOB units, scale is secondary to class of unit. For instance all OOB battleships are x-length… even if one battleship class is known to be significantly longer/bigger than a different nations’ battleship, they’re all the same length in-game. All cruisers are a little smaller than battleships… even though some newer cruisers were just as long as some older battleships… what class your unit is in OOB determines its length…
In HBG, unit scale is adhered to over in-game class scale. A Nevada class battleship is noticeably smaller than an Iowa class battleship (which in-turn means its much shorter than all OOB battleships). This is absolutely most obvious when comparing OOB tank scale vs HBG tank scale… while most HBG “medium” tanks are roughly the same scale as OOB tanks, when you start looking at heavy and light tanks they are all much bigger or smaller than any OOB tank. A good example of this would be comparing a Type-95 Ha Go tank from OOB vs the Type-95 Ha Go tank from HBG… they are both the same tank but the HBG tank (because it’s to scale with other HBG tanks) is WAAAAY smaller than the OOB tank… the OOB Type-95 is just as big as an OOB Tiger or Panther tank…
On scale issues, HBG planes (for the most part) mix well with OOB units.
IMHO, there is merit in both lines of thinking (all tanks are the same size, all battleships are the same size)… the method OOB units chose (all same class same size) I’m sure is for instant on-board game recognition, as there’s no confusing a battleship from a cruiser due to size, while the other method (all units are correctly scaled on a consistent basis) that HBG chose is certainly more realistic and may appeal more to those looking for realism over game function (though someone not familiar with ships might start confusing old battleships and newer cruisers as sizes are no longer the way to tell them apart).
I’m not stating the above to pass judgment on either method of doing things, each person will choose what they deem as preferable to them. I just wanted to mention if unit scale is something that is important to you (in either method), OOB and HBG do handle them a bit differently… some people wouldn’t even notice or care, others (especially OCD types) will notice the difference immediately (look no further than the Type-95 Ha Go differences).
Just putting that info out there for you if its important to you.