They are both cultivars and both like partial shade. I think these species are very sensitive to sun. The Garnet is probably more sensitive to sun then the Tamukeyama. In my case it means the garnet is more sensitive in that the spring growth is subject to damage by late frosts and too much sun (dry-out of the edges of leafs).
Ten hours of sun sound like a real challenge to me.
Piet,

Here is a link that might be useful: Deciduous trees in Tsubo-en

Non-nurseries like the plant departments at Slowe's and Home Despot tend to feature creative labeling. The batches of the two cultivars you are comparing could in fact actually be one cultivar.

For quality and selection, look to to see what good independent garden centers there may be in your service area. Japanese maples are extremely popular at this time. Independent outlets here display large selections including multiple cultivars of purple-leaved weeping laceleaf forms - in multiple sizes and prices.

I don't think 10 hours is out of the question for a Japanese Maple in zone 6, unless there's some other information we don't have. You'll get sun burn the first 2 or 3 years, but once established, and if you water deeply as appropriate, I think it'll be minimal.

I have to agree with bboy above that you should try and go to an independent garden center to make sure you actually get what you pay for. There's no telling if those plants at Home Depot are mislabeled or not.

Warren

Oddly, my best full sun performers have been "butterfly" and "seiryu" -- in my personal experience, the red ones seem to do worse (bloodgood, garnet, emperor 1..)
Tom