I just got a new road bike with disc brakes, so I can use just about any size tire.
I live in the white mountains and do a lot of climbing, and I will be riding the triple bypass in Colorado this summer. But I also do a fair amount of group ride we are on pushed to stay in a pace line with a pretty fast crew.
Just wondering if people think I’d be better off getting 25 or 28 Tubeless tires and why??
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28, hands down…especially if you are going tubeless.
More comfort, rolls fast, marginal aero penalty.
Just do it…trust me. 
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also more grip and less punctures due to larger contact patch & lower pressure
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No worry with the “weight penalty”??
Not in the slightest…any “penalty” is outweighed by the improced rolling resistance, comfort and handling.
Heck, I am tempted to try 30’s when my 28’s wear out.
Weight penalty comes when you go from 28 to 32. 25 to 28 is nothing to think about. 28.
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What rims are you using?
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28s, hands down. If your roads are bad, I would also consider going wider.
Not only have they lower rolling resistance, but they are also more comfy if you run them at equivalent pressures. Plus, they are also safer: if situation demands it, you can lower the pressure further to increase grip. I did this during a hilly race in the rain. My neighbor at the start line looked quite worried with his 22 mm tires and carbon rim brake wheels 
The only two microscopic penalties are weight and aerodynamics if you have (most) aero rims. Even if you are climbing a lot, weight doesn’t play nearly as much a role as people think.
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Mostly agree, but would point out that when people say that wider tires roll better, those tests are carried out at the same pressure. I.e. the same tire at 90psi will roll better in a 28 than a 25.
To get the handling and comfort benefits you mention, you want to be reducing the tire pressure, which increases the contact patch (more grip = better handling) and gives more suspension. This is why most people switch to wider and/or tubeless - the ability to safely run lower pressure without risking pinch flats. But…when you reduce pressure you increase Crr or rolling resistance. On a good road, a 28 at 70psi will roll a little slower than a 25 at 90psi.
Still probably a good trade off for most people most of the time, other than situations where speed takes priority over everything else (races and maybe fast group rides if you need every watt at your disposal to hang on!). And if you’re on rough roads then impedance losses can be higher than Crr losses, in which case that would push you towards a lower tire pressure for optimal speed. This article explains it much better than I could!:
https://blog.silca.cc/part-4b-rolling-resistance-and-impedance
Personally I run 25s on my TT and aero road bike for the reasons above (most of my races and fast group rides are on decent roads), and 28s on my winter/training/everyday bike for better comfort and handling. And am considering retiring that bike to trainer duty and upgrading to something that will take 33s or bigger.
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As everyone else has said, go with the wider option. And just to note if you have wide rims they’ll often measure even wider.
For instance I run Schwalbe Pro One 28 mm and GP 5k TL 28 mm, the former measuring 32 mm and the latter measuring 30 mm on my 20 mm internal rims.
As @cartsman mentioned you’ll properly run them at a lower pressure, I’m running mine at 4.0/3.8 bar (56/54 PSI) back/front (@ 70 kg), adding 0.5 bar (7 PSI) for short races.
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DTswiss PRC 1475
I ride 32s most of the time. No issues keeping up with a group and more comfort. If i was racing i would run 25 or 28 but for group rides, bigger is better.
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I run 25 in front and 28 in back - got the idea from @Nate_Pearson
I call it the “mullet” set up — business in front, party in back 
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That’s interesting, I might try that out. I remember going from 23->25 and it was clear there was no going back. 25->28 not so much. Not worse by any means, but I can’t tell the difference. Factoring in the weight, my weight (140lbs), the cost and I am definitely 25mm curious.
agree
Guys, would be the best choice focusing on comfortable and less tire puncture: a tubeless 25 mm tire or a 28 mm clincher tire?
I’m looking for a pais of Vitoria Rubino Pro G2, but the Tubeless 28 mm is about 40% more expensive…
I’m cycling now a 28 mm Vittoria Zafiro Pro VI with 75 psi nd sealant into the tubes.
Thank you!
The 25’s are cheaper because no one wants them…
Get the 28’s…more comfort, better traction and superior feel.
Tubeless sealant works by coagulating around a puncture, forming a seal. The lower the pressure, the better this works (higher pressures blast it out before it can solidify. And of course, the narrower the tire, the higher the pressure that most people typically run. General consensus is you need pressures typical of 28-30mm tires for tubeless to work well.
You can probably run TL 25mm at a lower (and more comfortable) pressure than a 25mm clincher, but not as low as a 28 mm clincher.
Unless you live in goathead territory, go with the 28mm clincher.
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Go with 28 mm. 25 mm is the new 23 mm. I use 28 mm for everything, including racing. If my frame supported 30 or 32 mm with plenty of clearance, I’d even ride that in the off-season.
Maybe I missed it but the riders rim width and weight should also factor into the answer.
A light rider with aero rims at 28mm or less outer diameter should consider 25s. Plenty of ride quality and more aero. If tubeless, you can run them ridiculously low pressure for comfort if desired. I see no meaningful benefit to 28mm vs 25mm tubeless at 160 lbs, and the 25s stay a bit under my rim OD as desired for aero performance. 28s won’t. But if I was 200lbs, or had really wide rims, then yea, I’d take the 28s.
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