A dual-band mobile covers 2m and 70cm, which handles the overwhelming majority of what most hams actually do on VHF/UHF. A tri-band adds a third band, typically 1.25m (222 MHz), and the question is whether that third band is actually worth the extra cost and complexity for your situation.
What dual-band covers
2 meters and 70cm are, by a wide margin, the two most active VHF/UHF bands in the US. Nearly every repeater system, most local nets, and the bulk of casual simplex activity happen on one of these two bands, which is why a dual-band radio covers the needs of the large majority of hams without any real compromise.
Where 1.25m actually gets used
1.25 meters is a genuinely useful band where it has active repeaters, since it’s far less congested than 2m or 70cm in most areas, but activity is regionally spotty. Some parts of the country have strong 1.25m repeater networks and dedicated user communities, while other regions have essentially no activity on the band at all. Before paying extra for tri-band capability, it’s worth checking whether your area actually has meaningful 1.25m activity, because a tri-band radio adds no value if there’s nothing to use the third band for.
Making the call
If you already know your area has active 1.25m repeaters, or you’re specifically interested in exploring the band, a tri-band mobile is a reasonable investment. For most hams in most areas, a solid dual-band mobile covers everything you’ll realistically use day to day, and the money saved is better spent on a better antenna or a second radio entirely. I’d default to dual-band unless you have a specific, confirmed reason to want the third band.
Related reading: Simplex Calling Frequencies and Etiquette · AllStarLink · Tropospheric Ducting