paris to strasbourg

When ?? -- that makes all the difference. What web site did you use for the price? That also can make a big difference. If the price was quoted in dollars, sounds like the EuroRail site.

There are several direct trains a day from CDG to Strasbourg. Walk up price is 95 EUR for 2nd class, which at current exchange would be about $238 US.
If you want to get a senior discount, you'll need to buy Carte Senior cards. It is an annual card costing 60 EUR per person and gives a 25% discount on full fare 2nd class tickets and 10% on Prems fares, and 40% on 1st class. That won't pay off for just this trip, but if you have many journeys, it might.
https://www.sncf.com/sncv1/en/discounts/senior-plus-card

You could also purchase advance nonrefundable tickets for a lot less, but a late arriving plane would void the ticket and you need to buy new full fare tickets. Suggestion would be to spend a night in Paris, then book advance tickets for a train the next day you know you can make.

And what time of year can make all the difference. August, all of France is on vacation and discounts evaporate like water in a desert.

The website I used was trainline. I am traveling May 15. Our plan was to take a train to Strasbourg and rent a car there but now I am thinking it might be better just to rent the car at the Paris airport. What do you think about that? Thank you!

There are trains from CDG at 6:53 (probably can't make that happen), 8:59, 11:21, 14:58, 17:43, 18:55. The 11:21 goes for 115 EUR pp, the others all for 95 EUR. It might be only 2 hours, but you're doing about 200 mph for much of it.

Renting a car comes with he standard caveat that you will be weary after an overnight flight and that could effect your alertness. There are 36 EUR of tolls to pay along the way, and you certainly won't be able to drive at 200 mph. Viamichelin says its a 4 1/2 hour drive.

French trains offer their cheapest tickets early, and when those sell out, they go to the next tier, and so on. Because of the current rail strike, the existing tickets on the days when the high speed trains are running are going to be in the higher price categories. It's unfortunate but that's European strike culture.

It's a two hour trip by train - but at least (probably more) like a 4 hour trip by car. I'd use the Via Michelin website to factor travel time by car, gas and toll cost, and then price out a rental car . Compare and contrast the price with the convenience and see which makes more sense for you.

I have done this route. I purchased the train ticket 3 months in advance for considerable savings. That said, I wanted to make sure to not miss the train due to flight delays. So I planned a day/evening in Paris and left for the Alsace the following day. If you have time for that and would like a day in Paris that is what I'd recommend.

Oh, and use the SNCF website, NOT RailEurope