I have to say Mike, this is one of the most difficult questions I have ever been asked! Both designs are flawless in my opinion plus the character themselves are damn diabolical my head can explode trying to think of who is better.
I will say the Showa design probably 98% because of respect for Tsuburaya's original design; plus, Showa KG has a more traditional and very Asian look to him which is a beautiful sight. His small heads with their beady eyes are just stunning to look at while the hair around their heads add a very natural look and feel to the design. It is a shame the hair concept was dumped for all future incarnations of the creature.
With Grand KG you have the most respect paid to KG's origins, something which, for some stupid reason was also dumped with all other KG's. My ideal KG isn't a mutated Furby or some ancient guardian Camel; he is a damn hellbent space dragon with no purpose in life other then to destroy all he sees. Something Toho did great with Grand KG. Had he had been in a more mature film Grand KG would probably be no. 1 by a long shot. The fact that he only attacks children kind of takes his badassism down a notch . Plus I have to agree with Arrow, KG's iconic high pitched cry (you can't really call it a roar) is missed with Grand KG,
however I am glad they replaced it. Grand KG is a much bigger and bulkier monster so to give this behemoth a high pitched cry would totally kill it. It works for the Showa because his character fits it perfectly.
While I must say I kind of dig the design of Grand KG a
tad better then the Showa design, I will have to say for overall character I will go with the Showa. and I'll probably change my mind after I post this
.