Definitely the buckeye, much more distinctive and novel. Colvos Creek grows it, think I saw some there recently. Also have one here from Oregon population via Cistus nursery.

california buckeye is available via mailorder from www.colvoscreeknursery.com (vashon island, wa) and www.forestfarm.com (in oregon). it is a beautiful tree (flowering now in california) but is early deciduous under drought conditions and needs some summer water to keep foliage looking good until fall. "marina" is a lovely small tree with evergreen foliage and handsome flowers and strawberry like fruit---it may be a little tender in cold exposed areas in the pnw and in california at least some gardeners think the fruit can be very messy. because it is evergreen and also has colorful smooth peeling bark, imho its a better all season/all year performer. the related a. unedo is more readily available and is quite desirable for the same reasons---though it lacks the pretty bark.

California buckeye never has problems looking clean all season up here, where we do not have 115F summers. After the large flower spikes (summer here), leathery buckeye pouches are interesting. During winter palish, partly penciled bark of specimens of some age and size is quite nice--two larger ones at Carl English Botanical Garden (Ballard Locks) really stand out when sun hits them (somebody cut down that Norway maple!).

All the characteristics listed above are why I have narrowed my choices down to these two trees. Thanks for confirming!

Any info on rates of growth? And will either tree resent periodic surface root disturbances if I underplant with perennials? At this point, I am leaning more toward the buckeye just because I don't know anyone else who has one (and I'm a sucker for roundish shiny things).

Here's a tidbit to consider when deciding between the two: CA buckeye flowers are toxic to bees. The larval food made from the nectar kills the larval bees but not the adults so honeybees, which are introduced from Europe, have never learned to avoid CA buckeye flowers. I have this from a 2nd generation California beekeeper so consider it reliable. I do not know if this is also true for other Aesculus species such as the red flowered buckeyes.

A friend had a whole row of California buckeyes growing in his hobby nursery near Bellingham some years ago. Didn't walk out to where these were last time I was there, don't know if they are even still there (don't get up there much anymore, they moved and rented the house out).

Bee toxicity is also mentioned by Thomas, ORNAMENTAL SHRUBS, CLIMBERS AND BAMBOOS (Sagapress/Timber).

Too bad Heronswood is gone, or you would be able to purchase the rare hybrid X Arbutsculus, combining the best features of both!

.....LOL