There are benefits to staying onsite at both WDW and US that are worth considering. At WDW you get easy access to your resort if you want to go back for a break partway through the day. This break is essential to include (for sanity and well being), especially when you visit during the summer. If you are flying (and since you mention using a taxi, I am guessing you are) then you would get Magical Express included for free (your transport to and/or from MCO to your WDW resort). Also you get access to the Extra Magic Hours - one hour early admission in the morning, three hours extra in the evening.
To take a taxi between WDW and US would be a bit time consuming - traffic tends to be heavy and there are lots of lights

- definitely $$$. Another thing to consider is that US has their own version of a Fastpass that allows you to go to the front of the line (and lines tend to be long...45-60 minutes), but unlike at WDW (where Fastpasses are free with admission

) US charges $40 per day for this

- unless you stay at one of the US hotels - then their front of the line access is included with your park tickets at no extra charge.
If you plan two or three days at US plus a few more days at WDW, for me I would stay onsite at WDW then move to a US resort (or you could even sandwich them WDW - US - WDW) The reason to sandwich them is you would get Magical Express to bring you both to and from the airport that way (town car service is ~$100).The US resorts are located within walking distance of both US and IOA. If you were planning to stay at a resort partway between the two, then you would probably be looking at renting a car for the length of your stay in Orlando.
For me it's not simply a question of saving $$ but also time. If you stay offsite between the two, you will spend a lot of time shuttling back and forth to parks, and you will be pushed to tour all the parks commando style (no breaks), since going back to your resort will take much more time out of your day. If you don't stay onsite, you will have to pay for onsite parking and your transfer to/from the airport - don't forget to factor these cost in (along with the rental car when you are pricing offsite hotels

).