Greeting!

Me and my girlfriend is planning a road trip from las vegas to new york.
We start in San Fransisco and will travel highway 1 down the coast of california to Las Vegas.
We'll spend 3 weeks travelling from Las Vegas to New York and we are looking for some advice.

First: which way would be most "scenic", we are wondering if we should travel the southern route through new mexico and texas or we should travel the northern route over dakota idaho and dakota.

We are planning to travel up the east coast.

Second: any tips on what to see.