It's really a question that is based on your own travel habits and priorities. If you don't travel much on Amtrak, it's not worth much. You get a few coupons, which are nice, and a 25% bonus point bonus but if you don't travel even $500 worth of trips, it won't add up to much.
I hit Select naturally just by my travel. For me, S+ is where it's at because I travel out of NYP which has a Club Acela and you get priority boarding from there. So thus it's worth it to get the card because it's what puts me over the edge. But $500 of Amtrak travel could be as little as a round trip on Acela or 5-6 trips on the Regional. I wouldn't care at all about TQP's if I used Amtrak that little.