Paniculatas are the most sun tolerant of the hydrangeas.

A third vote for a paniculata like Little Quickfire for reasons of sun exposure.

Little QF...two LQF between a PG

How about Limelight bush or tree? My tree gets sun for a good part of the day up to early evening. It takes the sun well and provides shade for other shade plants underneath.

What area are you in?

Keep in mind that one person's Zone 5 full sun is not the same as another's. Full sun in my area is very intense, hot and dry. More like high desert. Many plants that will take full sun in the midwest or East where elevation is lower and there is more humidity will just outright fry in full sun here.

I'm in NW Illinois, Zone 5b.

Lucky you! I think NW Illinois will be hydrangea heaven! I'm sure either one of your choices would be lovely. I'd probably have a hard time choosing between the two myself. I really love the look of that Invincibelle Spirit II. When I compare images of the two, the Invincibelle has more of a traditional/formal garden feeling where as Little Quick Fire seems to have more of a cottage garden look to me. Do you think you gravitate toward one or the other style?

Even in my cool, moist part of the US, I give my H. arborescens afternoon shade. Mine is in sun from around 10-2 and then is shaded by the house. I don’t want to have to constantly monitor it for moisture, and IME even short periods with too little moisture lead to the leaves and flowers browning early.

One more thought triggered by popmama’s comment on IS II looking more formal. Being in the Midwest, I imagine that you are prone to periodic severe downpours. IS II will like all arborescens flop some after heavy rain if not supported. I would expect little to none of that with LQF.

Thanks everyone for your advice. I have 2 or 3 IS II in pots that I'm hoping to plant out in the next few weeks. I'll have to find a spot for them in the garden. HD had them on sale a few weeks ago and I was able to get them a good price. As for my LQF hydrangeas, those are in the ground, but need to be moved as they are being crowded out by perennial plants. I'll probably move them next Spring.