tri-flex underlayment instead of felt paper?

After a 10-year interruption, we're re-roofing the last side of our (hip) roof. The other three sides are done with asphalt shingles over felt paper. Now, my brother the roofer says he uses "Tri-flex 300" underlayment, instead of felt. His reason for that seem to be that it doesn't tear, so (a) it's easier to put on, and (b) it seals better around nail holes. The only reason we have not to do it is that we already have the felt on, and it's not clear that the Tri-flex will really matter.
If anyone has any experience with this, I'd appreciate hearing of it. The roof pitch is ~40 deg, and we get snow in winter, sometimes quite a bit.
TIA, George
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