Desiros anytime. Yes the air con can be a little noisy and yes, the ride on some of the 450s can at times feel a little stiff but. Unlike numerous trips on Electrostars I have never had a trip on a Desiro that hasn't had no air con, uncofortable of felt badly built. For example, what I dislike about the Electrostars are - the pointless overhead reading lights that don't match up with the seats and offer nothing, the awful ride quality over the bogies, the dire interior layout in some of the units - you can easily walk down the isle of a packed 450 with its 3+2, it is t as easy walking through a 377 in the evening peak as I've had to find out several times over the past week, the overall feeling of cheapness across the electrostar family - impressively none of SWTs Desiros have yet received a refurbishment, not because they need them but because of good quality and preventative maintenance. Unlike several of Southerns Electrostars for example, some of which are seeing their second and third refurbishments come up. And the Electrostar air conditioning has never impressed me either, the worst was a journey to an apprenticeship interview - on a 375 with no air con, on the hottest day of the year. Having to buy a replacement shirt before the interview did not leave me impressed! And that's all without mentioning the curse of the missing door buttons...
But also, and ultimately reliability also does it for me. Remember when we had all those winter snow storms? SWT ran a revised timetable but to a near full day. Southern and South Eastern both could not due to rolling stock issues. Another plus point I will have for Siemens is that the Desiro Mk1 fleet were built from 2002-2013, with Siemens working on they're replacement as far back as 2010 - Bombardier on the other hand are stuck in the Model T ford principle, you can have any train you want as long as it's an Electrostar - 1999 - 2017, it just doesn't look or feel fresh anymore, no matter what small upgrades Derby tries to stuff inside it. The desiro fleet feel like they're designed and built by Engineers. The Electrostar fleet, does not.
Of course it all remains to be seen how the Desiro City's will fare against the Aventra, but my money's already feeling safe on one!