Did you ever get a comparison of the Tumbler versus a Rhino? I'm thinking of getting a couple for my new Space Wolves army... but I fear I will just regret it and get rhinos later and then that's a big waste of money...
While I think I posted some dimension measurements, I never actually took any side-by-side pics - until just now. Here they are.
With this first comparison, I lined up the backs of each vehicle and you can see that the Tumbler is a few mm longer than the Rhino.
For a width comparison, I lined both vehicles up on their left side. The picture makes the Tumbler look considerably wider because of the handrails, but I hope you can see that without the handrails, the Tumbler is only 3-4mm wider than the Rhino.
Finally, comparing the heights was easier, lining them up from the bottom of their tires/treads. You can see that the Rhino is about 5mm taller than the Tumbler.
I've finally got five assembled. The first, the original, is fully painted. Two have been Games Workshop-ified with the addition of Space Marine and Imperial Guard hatches and vehicle bits. The last two are sitting waiting for me to pick up some more Imperial Guard hatches to make them consistent with the first two.