Triples work great with closer spaced cassettes like a 12-26.
You said you ride "flats, rolling hills, big descents". This 12-26 will be perfect. And you can still do some very steep climbs with it.
(Upgrading? Good tires like GP4000S are worth the extra money. Their flexible sidewalls smooth out rough roads, use less pedaling energy, and they have great grip in the turns. Use an appropriate lower air pressure for your weight, don't go 110 psi.)
Mike Sherman's gear calculator shows it clearly. (When you click the links below, a box pops up to tell you that you can save those settings as a favorite/bookmark. Just click OK.) The calculator changes it's charts on the fly if you change rpms or cassette sizes.
Here's your current 30-39-50 triple with the 11-28 9-speed cassette. Each chainring is a different color, 30-red, 39-black, 50-blue.
See this link to the
11-28 setup on the calculator.
The Speeds at RPM chart:
And see this
12-26 setup.
Compare the gear spacing between 10 and 20 mph. The 12-26 has closer spacing, which is better when you are trying to get the "just right" cadence, not too hard or too easy. It loses a few mph of top end speed over 32 mph, but you can just coast downhill at those speeds anyway.
The 30-26 low is about 8% harder than the 30-28. It's about
halfway between your
current 30-28 and your next highest gear,
30-24.
At a slow 60 rpm, the 30-26 is 5.4 mph instead of the 30-28's 5.0 mph.
If you sometimes climb very steep hills in the 30-28 and are struggling, and wish you had one more shift, then you might want the wider range cassette. But otherwise, go for the 12-26.