Overall, yeah, probably, because you aren't paying sales tax ... but the installments on the device should be the same because Apple makes you pay the tax at the time of purchase... and the 24 monthly payments would be on the actual price of the phone... so they should be the same.
Absolutely. Even with payment plan you pay all sales tax up front while your monthly payments are not taxed. Pioneer Square Portland Apple Store have more people from CA than local anytime of day because the oppressive taxes in CA.
You can also walk down the street to the LV store and get a nice new case tax free!