Edinburgh up to Inverness

It's 180 miles each way, which is 4 to 5 gallons of petrol/gas- £24 to £30 each way.
You should get an advance train fare of £18 each (£13 each if you're lucky) each way, and both get to enjoy the scenery without one of you having to concentrate on the road.

So, if you want a break from the driving, on a purely money crunching basis, it makes sense.

Thanks for your reply. Financially it certainly does make more sense and most importantly, much more relaxing. I'd much rather take the train, even though it means returning back to Edinburgh for an additional night. We will then be set up for an easy transfer over to the airport late next morning. We have a longish travel day (once we arrive at Shannon we are getting a rental car and driving up to Westport).

An additional ? ........does anyone know if it's possible to take the train into and out of the Inverness airport? I'm assuming we might have more car rental choices there. Although, I understand that some car rental companies would meet us at the train station in Inverness.

Thank you!

Yes hourly (sometimes half hourly) Inverness to the new Inverness Airport station (opened in February)- trains to Aberdeen.
You can book a through train ticket from EDI, even a through ticket which includes the airport shuttle bus.
It's a 12 minute walk on a dedicated path from the Airport Station to the car rental counters. Also a regular shuttle bus meets all trains.
Three car rental counters at the airport.
The cars are straight outside the terminal.

See this thread- https://community.ricksteves.com/travel-forum/scotland/inverness-airport-railway-station

You don't say why you are going to Inverness - that might decide whether to drive or train. If it's just to go there, I would train it. But without a car it's a bit tougher to explore that area

I wanted to use Inverness as a base for the short amount of time we will be there, 2 nights. I've been before, but my dear childhood friend, also travel companion on this trip, has not. I thought we'd stay in Inverness, or nearby, and use the car to go to Culloden (I'm a Gordon and found Culloden heartbreaking but so historically significant that I am excited to be visiting again-it's been 11 years since my last visit). We would then tootle around to some other sights with our remaining time. We will be spending two weeks in Ireland after this short visit in Scotland and will be doing lots of hiking and lots of outdoorsy type of stuff in County Donegal, Westport and Ennis. This is the reason why I didn't want to go too far afield (eg. Isle of Skye, Oban, etc.) and have similar experiences to what we will be doing while in Ireland.