Mainly the difference is dynamic explosiveness. The Diana Integrated has a bit more of that (but really only a bit) as you might expect from a larger power supply and larger output transformers. Tonally, they are very similar, despite Diana being all DHT...it might be a bit "creamier" but only very slightly I would say. Transparency is essentially equal as well...Genus might even win here. On a dynamic (i.e. not horn) speaker, the Diana integrated might have a bit more bass grunt, which could help if it is used on speakers with harder to control woofers (low impedance). The tradoffs of reducing the size of the amp from Diana size to Genus size are very small indeed. Of course if you consider 25 watts sufficient and want to go to the max then the Concero 25s would be an interesting choice. These are about 70Kg each, so even more ambitious power supplies than Diana with the same simplicity of 2 stage circuit like Genus (this also makes a lot less heat).
That said, we used Genus at our recent show with tiny Boenicke W5s (which are only around 83-84db sensitivity) and Kassandra and people were slack jawed at the HUGE and dynamic sound this produced...it even surprised us, given that Genus is "only" 25 watts. We were playing quite loud as well...no one could believe their ears and eyes (there was serious congnitive dissonance at the discrepancy between size of speakers, amp/DAC and sound).