I've only ever done Master Grade kits but while looking for stuff from Gundam Seed I noticed there was a lot more models in the HG line (a whole Seed-specific line in fact).

Just how much "lamer" are they compared to the typical MG kit?

HGs aren't generally as detailed as MGs (mainly due to scale limitations), but saying that a lot of the newer HGUC kits are excellent kits. As for the SEED kits, a few of them are quite average but there is the odd one that is excellent such as the Red Frame Astray.