Did the 2.5 hour and it was fine. I don't know where they take you on the 8 hour trip or what you are going to see, but any excursion of 8 hours is just too much for me.
The scenery is magnificent and as CA Cruiser said, the seats are not comfortable and it's not a smooth ride.
From reading the Alaska brochure on-line, on the 8 hour trip you travel by motor coach up to the Yukon, stop for a BBQ chicken lunch. You then return to the ship on the train. The brochure states
"Please note: As this adventure crosses the US/Canadian border, you
must bring the following identification with you: US & Canadian
citizens must carry a passport, or valid driver’s license and birth
certificate, and carry their ship ID. All other nationalities must have
a valid passport, visa and ship ID.
If you do decided to do the 8 hour trip, make sure you take your passport with you.
Forgot to mention that the 2.5 hour train trip is over $100 pp, so the 8 hour one will be very expensive.