Thanks for the tips everyone!
I think both games look fun and are value for money, so I've bought both! I've only played for about 30 mins each so far, but here are my initial thoughts:
Spinout
Great graphics, audio and controls. Quite challenging to navigate the ball around the tracks. Nice use of physics and special effects. The game has a very Wipeout-meets-Mercury feel to it, which I love! :-)
Super Monkey Ball Adventures
Great graphics & controls. The story mode stuff is an interesting diversion but grows old quickly for someone my age (but I can see how it would appeal to younger players). The actual mini games are quite challenging. The ball's inertia and the 3D view can make it tricky sometimes to accurately position the ball, even though you do have some control over the camera and there is a small overhead view on screen at all times to help with this.
Summary
Both games are good buys if you like these sorts of things. Super Monkey Ball Adventures is kind of what I was expecting it to be to be. Spinout on the otherhand is much better than I was expecting and is defintely the better game out of the two. It is also better priced, which makes it great value. As igotmy9milli said above, there is a lot of gameplay in Spinout.
So once again, thanks everyone for your help & suggestions.
That's two more great games added to the "Volcane Game Library"! :-)