I received a request on Benno's Figures Forum, where I posted about my African Mercenaries with the blue shields, about comparison shots between Hat 1/72 figures and the Newline Design 20mm figures. I am popping them here in the workshop for future reference.
I have said before how I collected 1/72 Ancient miniatures initially following army lists in Field of Glory supplements. However I looked into filling blanks at the time with Newline Designs 20mm lines, namely the Thureophoroi and I asked about African Veterans in Roman armour. After receiving the metals I loved them and over the years I have been collecting Newline miniatures with 1/72 plastics as secondary.
Newline miniatures tend to be chunkier than the Hat plastics depending on the sculptor. Earlier Newline miniatures seem noticeably smaller than 1/72 and I think that as quite a lot of people asked Sean at Newline about how close they were to 1/72, later miniatures were made a bit larger and fit better height wise.
Some Hat figures appear to be full of action, mid-slash, running throwing etc. and some of the earlier Hat Ancients have shields cast on which lead to flat looking and sometimes awkward poses. Newline poses tend to be more conservative with most heavy foot either standing or advancing tucked and hunched behind their shield. These factors also accentuate the size difference.
I generally don't mix manufactures on the same base/unit but I would definitely mix within an army. I have seen them mixed within units and I think they mix well when painted and based in a consistent manner.
Some examples I have seen around the web: