Save money, buy a WiFi only tablet. Many Apps for smartphones now will allow you to create a WiFi personal hot spot, then just connect your tablet to that 
You might want to get a big/unlimited data bolt-on for the phone 
Have you read any of this thread, Tunnie?
Yeah, but is the tether bluetooth or WiFi? I have no issues with speed if I do a WiFi hotspot on my iPhone to my laptop, decent browsing speeds if on 3G.
Assuming your tether is good, its no faster having on-board SIM on the same network.
Right. I understand. 3g crap on phone=3g crap on tab. Hmmmmm......
Especialy if you use the same SIM provider for both ....
You may wish to consider using a PAYG sim from a different provider in the tablet .. gives you more options ..
If phone provider is poor, tablet provider may work... if tablet provider poor but phone provider good .. then tether ..
Obviously you have to make sure the "other" provider is truly different, I use vodafone on one device and gifgaf (O2) on the other .. so far one or other has always worked .. although 80% of the time they both do !!!
