Tamiya Shermans are good builds.
They have probalby the best set of pioneer tools available for any currently made Sherman kit.
The solid style road wheels and idlers are weaker than those supplied by Dragon (no greese plugs on the outside surface). But both Tamiya and Dragons solid style road wheels are behind Academy's in quality (Academy has detail on both sides of the roadwheel).
The main problem with both the Tamiya Shermans and the Dragon Shermans is the VVSS suspension. Both are accurate for a Late Version VVSS. What was much more common on the WWII Shermans is the Mid Version VVSS. These can be substituted for Academy or AFV Club parts, both of which represent Mid Version VVSS systems.
I can only echo that you will need to fill in the sponsons with some plastic stock on the Tamiya kits.
Dragon copied a lot of their parts from Italeri and as a result, copied the errors in the exhaust system on the M4A3's. And their pioneer tools aren't as crisp as Tamiyas.
It really comes down to what Sherman do you want to build. The versions offered by Tamiya and Dragon do not cross one-another. Tamiya offers a very late 75mm M4A3 (Late VVSS is perfect for this kit), an early 75mm M4, and the 105mm M4A3 (Again, Late VVSS is OK for this kit). The Dragon kits don't cover these version. Their 105 was with the E8 suspension, they offered several 76mm versions, and the early Sherman is an M4A1 not an M4. Dragon also added an M4A4 (applause!), but got the lower hull length wrong (it's too long and need to be cut down). Dragon kits also come with link to link tracks. A positive for me, but a negative to others.
Stay away from the early M4A3E8 offered by Tamiya. It's not up to par anymore.
Be aware of the older Dragon M4A3E8 releases. They are buildable, but require some cutting on the lower hull to get the suspension in place.
I hope this helps.
Have fun whatever you choose!