Well, if you already have an iPad then decide on the following:
1. Battery life - how important is it? It looks like you read a lot, but if your iPad battery lasts long enough, than this is not really a factor.
2. Matte paper-like screen - if you read a lot in bright sunlight, or just prefer reading on e-ink, then Kobo might be more pleasant.
3. Other factors are weight (kobo is significantly lighter, I own one and love to carry it around) and the fact that kobo's memory can be expanded with a micro SD card.
If none of these matter, I would probably stick with the iPad. I primarily use my ereader for the first two factors. I charge my kobo once or twice a month and the battery last long enough to take it to long flights/trips. Plus reading is more eye-friendly, although it lacks back light.
Hope this helps.