For G2:
D802: International model
D800: AT&T US model
D803: Canadian model
LS980: Sprint US model
D801: T-Mobile US model
VS980: Verizon US model
L-01F: Japanese model (JDM lol)
For S4:
GT-I9500: Exynos International model (3G)
GT-I9505: Snapdragon 600 model (4G)
GT-I9506: Snapdragon 800 model (4G+)
SGH-I337: AT&T US model
SCH-I545: Verizon US model
SGH-M919: T-Mobile US model
SPH-L720: Sprint US model
SCH-R970: US Cellular
GT-I9508: China Telecom
GT-I9502: Dual-SIM
SCH-I959: China Telecom
SC-04E: Japanese model
SHV-E300/SHV-E330: Korean model (S800) (4G+) (almost identical to I9506)
Pretty thorough list of all models.
For G2, D802 is the best. D800 is okay but ZONG/WARID 4G won't work on it.
For S4, TBH all are fine.
Specs of I9505 and all US models are the same.
And I9506 and all Korean models are the same.
But stay away from the Korean, Japanese and Chinese ones.