I currently use ~17GB on my 32GB iPad, and ~12GB on my Evo. The amount of storage I use on the iPad hasn't increased much over time, but storage I use on the Evo's microSD card fluctuates a lot, since I'm always adding and removing music and podcasts.
Since I'm going to switch to the Nexus S when it hits Sprint, I'm going to buy the 16GB Transformer and give it my Evo's microSD card. If I need more than 16 gigs on the Nexus S, I'll offload the excess to Amazon Music Cloud.