Argonians are great but Bretons have strong passives that aid in sap tanking.
For example the 4k spell resist means that you don't have to wear spell mitigation armour to get close to the cap. It allows you to run a set like 4 piece allesias bullwark and a 5 piece damage set like julianos or kagrenacs. 33 000 mit is the cap but if you get to 30 000 buffed you are doing just fine. Every 650 mit is 1% reduction.
The 3% reduced magic cost is great paired with the CP since the cheaper we get our swallow souls the more times we can spam it to proc Siphoning attacks. My light attack siphon return is more than the cost of swallow soul. So if Im LA weaving soul its basically free to cast.
10% more magic allows you to not only have a larger pool but increass your damage and your shields. More damage means stronger heals.
Argonian if only for the poison resistance. The extra healing probably works out the same as an extra 10% magicka.
No race, a sap tank is probably the most useless build you can bring to PvP. If you want to play a tank, Dk and Sorc provide way more utility.