GoKawiilAs noted, the shape and size is the biggest difference between these two phones, which otherwise share a lot of the same specs, including a new, more durable Flex Titanium display with a less noticeable crease and better flexibility. The Z Fold 8 measures 123.9 x 81.9 x 9.7mm when it's folded and unfolds to 123.9 x 161.4 x 4.5mm. It has a 5.5-inch cover screen and 7.6-inch main inner display.
By comparison, the Z Fold 8 Ultra is taller and slimmer with a 6.5-inch cover display and 8-inch inner main display, so it's larger overall. That makes it similar in size to the previous generation Samsung Galaxy Z Fold 7, which we consider to be one of the best foldable phones. The Fold 8 Ultra measures 158.4 x 72.8 x 8.9mm when folded and 158.4 x 143.2 x 4.1mm unfolded, so it's thinner overall as well. The Ultra variant is slightly heavier, though, at 215 grams versus the Galaxy Z Fold 8's 201 grams. It should be noted, too, that both screens on the Z Fold 8 Ultra have a higher resolution than their counterparts on the Z Fold 8.
Both phones come in Graphite and Cream, and where they differ here is in colors beyond that. The Galaxy Z Fold 8's additional color options are Lavender and an online-exclusive Pistachio while the Z Fold 8 Ultra's are Violet Shadow and a Green Shadow online exclusive.
Design is the biggest factor that will influence your choice. If you want a bigger overall screen, on both the outside and inside, the Galaxy Z Fold 8 Ultra is the best option. However, if you found the outer screen of the Galaxy Z Fold 7 to be too narrow, you might like the Fold 8's wider external display with the Galaxy Z Fold 8. It's also smaller overall, though it might not as easily slide into a pocket.