What is your entire itinerary? How else would you get to Seward? How long would you stay?
I have taken both the train and a rental car to Seward. The train veers off from the road for a while and you see a glacier in the distance. So it can be considered slightly more scenic. However, the train doesn't stop every time you see something you like and want to look further and/or take a photo. I have seen sheep and moose along Turnagain Arm and often stop for that or just the scenery. Also, there is so much to do/see along this route that if you drive you have a chance to do/see much of it. Potter Marsh, the turnouts for scenic viewing of the water and mountains, Girdwood (Alyeska tram, "hand tram" and more), Wildlife Conservation Center, Portage Glacier area, the cute town of Hope and more. It often takes me 4-6 hours to get from Anchorage to Seward depending on how many stops I make.
A key reason for going to Seward is to take a Kenai Fjords National Park boat tour of 6 hours or longer.
I would echo what FTI said. We just got back from our honeymoon and took that train. We had 2 days (1 night) in Seward, took the train to and from Girdwood. The train route is scenic, but the road route is also very pretty.
On the train we saw 3 black bears and 2 moose. But all of them were hard to take pictures of as the train was moving fairly quickly. However the route still takes longer than taking a car with multiple stops. If you do take the train, the upgrade to goldstar is more than worth it.
The Wildlife cruise we went on in Seward was exceptional! I recommend staying at least one night if doing the bay cruise, as train-cruise-train is a very tight schedule.