Well, a good 4G Wimax signal is always going to trump any 3G connection. 1-3mbps vs. 3-10mbps(I've seen as high as 16mbps on Sprint 4G).
As the Galaxy Tab only has a 3G EvDO Rev A radio, keep the overdrive if you want the fastest tether speeds. Or get ride of that iPhone and get an EVO 4G and use its hotspots feature. You want 4G connectivety whenever possible.