Herc is definitely bigger. Way bigger. I have neither, but personally find the Giant bots to look better both individually (alt modes are more realistic, although they do lack some of the functionality of the Herc ones) and combined. I like the look of the colours better as well. Price is a bit easier to swallow too.
BUT I'd definitely recommend that you look up some reviews of them both on youtube and other websites, and see which set appeals to you more. TJ Duckett aka Kuma Style Reviews has done some fairly in-depth comparisons between the two, and he's not the only one. Also there are a lot of reviews for each set on their own. See which YOU like better, and if Herc is the one you go for, you can always save up for a bit longer.
assembling a Neo-G1/TF:TM cast. Please PM if you have (or know of) the following at a reasonable price: Classics or Henkei Prowl, Astrotrain, Ramjet, Skywarp, Sunstreaker, Sideswipe, Blurr, Perceptor, GDO Wheelie and Springer, or 3rd Party Motobot Arcee, FP Springer, iGear Ratchet and Ironhide, City Commander (and/or Ultra Magnus), Starscream (deluxe) Coronation add-on, or Spike and Daniel in their Exo-Suits.