I've done the tubing and it was great. I haven't doen the rafting, so I can't make the comparison for you. I will describe the tubing, if that'll help.
The tubes are large inner tubes with bottoms in them - so if you float too shallow you won't scrape your own "bottom" - I really liked that part!
There were several small rapids in the river and it was exciting enough for me being in a tube without any paddle. I've done rafting at other locations and love the excitement of the big rapids, but I wouldn't want to do that without a paddle!
Most of the time, we just floated - it was quite peaceful and calm. The area was shaded well, so it wasn't too hot. The guide pointed out local flora and fauna on our way. We got out of the river about half-way through to get a drink (extra money here) and see some of the flora up close.
We really enjoyed ourselves - it was just the three of us in our group.