These solve different problems at very different prices. The R5 is a complete starter kit - base, ES wheel and SR-P Lite pedals - for $399, the answer Moza needed at the entry tier. The T818 is a 10 Nm base alone at nearly four times the price, and its real draw is the SF1000 F1-style rim catalogue.
So if you are starting from nothing and want value, the R5 gets you driving on day one for a quarter of the money. The T818 is the pick only if you specifically want the Thrustmaster ecosystem or already own its wheels - as a value proposition against the R5, it does not compete.