Money: Bulb has a standing charge almost double that of Iresa, at 25p/d. Sure, Iresa has infamously crap support, and some here are attesting to Bulb’s being great, but bear in mind you’re paying nearly £5pcm for it. (Worth it to some, and not to others, of course.)
(The unit charge is slightly more too, 11.75p/KWh vs. 11p/KWh.)