Poly limits in a given mission featuring an arcadia wouldn't be broken by a few more polies in the hex section, but making that section nice & round would have looked a bit out of place when all the other ships at that time were pretty blocky overall, terrans in particular. Back in that day, polycount in a certain scene could get out of hand in a hurry with the average hardware on a pc of the time. Plus, in this situation, to me it's a showdown between "rule of cool" while at the same time avoiding straying too radically from the original design. Staying with a hex-shaped center satisfies both of those, as I know without a doubt that maintaining that shape doesn't at all mean a new Arcadia is going to look any less great for that reason alone. However, if the only thing on the model different was a circular opening as opposed to a hex, that won't come anywhere close to making it unidentifiable for me. Once again the best idea is one that's already been stated... make one of each.