What is the size differance between 1/700 and 1/720?

The math is

(700 - 720) / 700 = -.02857  ~ -3%  (1:720 is about 3% smaller than 1:700)

      or

(720 - 700) / 720 = +0.02777 ~ 3%  (1:700 is about 3% larger than 1:720)

As a matter of personal preference I try to not mix scale differences in excess of 10%.   An example of this would be mixing 1:400 and 1:350.   (350 - 400) / 350  = -0.1428  (1:400 is 14% smaller than 1:350)  and (400 - 350) / 400 = +0.125  (1:350 is 12.5% larger than 1:400).

The choice is yours

 

 

Well, 1/720th of a foot is 0.01666" and 1/700th of a foot is 0.0171", so the difference is a bit less than a thousanth of an inch per foot.

But, such differences are only the digits.  Some kits are "finer" to the eye than others, this can 'defeat' their similarity in scale.

Like as not, your best bet is to not have vessels of similar type at differing scales where they are close enough to compare.  So, I think (just me, others may differ) you could "get away with" having, say 1/700 escorts around a 1/720 carrier as a for instance.  Maybe.  Depending on which kits, etc.

Because, that will be the "rub" of it.  That 70's era Revell "1/720" carrier (which might be, oh 1/728 or so) might not 'group' well with a new Trumpter 1/700 DD.  Or it just might, kits vary.

Scale will/can "getcha" too.  Underway replenishment is about as close as working naval vessels get without Courts of Inquiry.  UnReps are about 150 to 200' of seperation--that's about 3" in 700/720 scale.  Even 'closed up' as close as 1000 yards, that's 50" to scale.  If we postulate that we can "crowd" a diorama a bit, even a couple of feet of seperation will 'blur' scale differences a bit.

 

 

I would agree with CapnMac82 that it depends on which kits you're planning on grouping, and under what setting. That said, I put my half-done 1/720 Revell USS Enterprise next to a finished 1/700 Dragon Perry-class frigate, and it looked about right.

A lot of it depends on your audience. If you're thinking of entering a grouping in a modeling contest, I would think very hard about any kind of grouping, and pay attention to distances because, well ... you know how anal us modelers can be . If, on the other hand, you're doing something just for you, or for a friend who doesn't know any better, go ahead and do what looks best/most interesting to you! The differences is scale are not going to be noticeable enough.

Someone once told me that one is an architectural scale and the other is an engineering scale.  As to which one is which,  I don't know....  Some help, eh?....

1/720 is 1" =60' so I suppose that is the architectural scale. 1/700 is more conducive to dividing decimals, ie. engineering type dimensions. Although I guess that these days with everything done on computer, it doesn't matter much.

if you were to use the scales together in a diorama for perspectives sake you would put the larger scale in the back (that is depending on what ship is what and how out of scale one may look)