Is TriPeaks or Golf harder?

TriPeaks, and by a wide margin. Golf looks easier because every card is visible from the start and the layout is flat, but the missing Ace–King wrap is decisive: Jan Wolter's solver analysis of 100,000 deals found about 45% of Golf deals winnable with perfect play, against over 90% for TriPeaks. In TriPeaks a chain can always run round from King to Ace; in Golf it stops dead.

Should I always take the longest available chain?

Not automatically. A shorter chain that clears a peak or uncovers two cards often leaves the board in much better shape.

Is there any point tracking which ranks are gone?

Yes, though less than in Pyramid. If all four 9s are played, an 8 can only ever accept a 7 — worth knowing before you commit to a line.

TriPeaks Solitaire Strategy — How to Win More

TriPeaks and Golf StrategyTriPeaks rewards looking ahead: check the whole chain before starting it, prefer cards that uncover two others, and keep both directions open.