It's about the size of the pans, mostly. A 1-litre pan is fine for boiling water for two cups of Brown Drink, and about right for cooking a single large portion of amorphous rice-y pasta-y stuff (remember, you need enough space to stir it without spilling).
So, it depends exactly what and how you're cooking. You can obviously do two one-pan meals on a 27, or a two-pan meal if it divides up reasonably well, but you probably won't get enough pasta for two decent portions of pasta'n'sauce out of a single pan, if you see what I mean.
For simple boil-it-with-water meals, cooking in series with a 27 is a legitimate technique. It's not like you're usually in a hurry while camping. Probably not so good if you're chopping up an assortment of meat and veg for something posh. If you're frying, then the smaller area of the 27's pan is going to be quite restrictive.
Looking a the specs, the size difference doesn't actually sound like all that much, and presumably you can make use of the increased volume to stash more of your cutlery/condiments/washing-up kit inside the 25. I don't think I'd mind touring solo with a 25 series (I have a 27), given that when I'm being really lightweight I tend to leave the Trangia at home and carry a basic gas burner.