Not an expert for sure, but I have been to both.
Ivrea smaller, so perhaps ideal for a half-day, with a nice quiet pedestrianised main shopping street, and a super restaurant in the form of
http://mugnaia.com/ down a few steps from that street. Do grab some grissini from one of the panetterie as well, very much a regional speciality. Perhaps other stuff that takes your fancy, but definitely a 'bite sized' location
Novara bigger, and something of an under the radar charmer, with plenty of architectural appeal, including work by the person who designed Torino's Mole Antoniella. There's a famous biscuit shop
https://www.camporelli1852.it/ I could see it being appealing on a Sunday as it would be lovely to walk around when quiet. In such situations I'll sometimes see if there are events on, and it seems there is a 'secret' guided tour of the basilica.
https://www.turismonovara.it/it/eventimanifestazioni?n=4 which might make for a different experience.
In short, both could appeal (as might vercelli), though maybe I'd lean towards Novara
Another option might be Gattinara, which is a modest sized wine town, maybe a fraction smaller than Ivrea. Some decent history there, plus I love the wines! Antoniolo may not be the cheapest, but are very serious about their work. Travaglini an established name with quirky bottles (a retro re-imagination of older bottles). The wines good and approachable.