The Ideal is much quieter. Not silent, but quieter. Like I'd bring it to a sex club, and would not bring the Hitachi (other than the plug in factor).
I'd say most toys are about a 3 or 4, maybe a 5 if they're good. The GiGi (by lelo - LOVE it!) is maybe a 6 or 7. Ideal is an 8, and the Hitachi is a 10.
I mean, it is a trade off, and if you can afford it, I suggest both. I have 3 Hitachis (one for home, one for travel, and one for just in case), and one Ideal. The Ideal I use for travel to to partner's houses; I find it is much better for use during partner sex, as it is quieter and less unwieldy. However, during solo sex? Hitachi, hands down (literally).