I would think that a renting a large car would be a more affordable option, but it may
depend on how large of a car
In your search to rent an SUV or
your best available option -- it can be difficult to find availability for SUV's on a oneway rental. As far as the time frame, a seven day one way car rental will be less expensive than 10 days.
I don't provide quotes (prices change too quickly) but I checked for rate examples for the first week of October to see if I could answer your question:
to rent an SUV or Large car for a one way trip from Phx, AZ to Arlington, VA
For 7 days, the full size car was cheapest , a premium car was next with the SUV being the most expensive.
For 10 days for the full size, SUV or Premium car the price was almost $1000 more for 10 days than for 7 days. That's a significant increase for 3 extra days. (For 10 days, the price on the premium car and SUV was the same, with the full size car being slightly cheaper).
So, for Avis, 7 days is definitely a better deal and the SUV would only be about $150 more than a full size car. You have to weigh the cost difference against your vehicle preference and the extra gas costs for an SUV.
When I checked other car rental companies (links open in new windows):