This is a question I can get asked a lot by people who want to commission a pet portrait. It's a tricky one to advise on to be honest and it depends on so many factors! I have even had a client have both, a mordern pet portrait and a realistic pet portrait because she couldn't decide!
I think so much if it is simply what style you prefer! It really can be as straightforward as that. Perhaps you have very contemporary decor, and then a deep edge canvas with my brightly coloured and modern pet portraits maybe more up your street. If you have a lovely country cottage then perhaps something more traditional, but I truly wouldn't base it solely on the decor of your house, that may change and actually both styles can work anywhere.
It is worth bearing in mind, my pastel pet portraits cannot be painted on canvas, they all need to be framed behind glass to protect them, whereas the more modern watercolour portraits can be either painted on paper and framed or painted on deep edge canvas, so if you prefer something on canvas this maybe your better option. I don't frame my canvas work, but should you decide to get it done at a later date it can me done at your local framers.
Perhaps you one particular style you feel sums up the character of your pet better. If you have a quirky dog with a crazy character, perhaps the contemporary watercolours will suit their character, but again that's not to say the pastels won't either!
Another factor is budget. My modern pet portraits do cost less, this is not to do with quality or even quality of materials used. I always use the highest quality and most lightfast I can to ensure the longevity of your pet portrait, it is down to the time they take to produce. Pastels take a lot longer due to the amount of detail that goes into them. The watercolours are a lot more spontaneous and therefore quicker to produce, but then this is what creates their quirky character!
So really apart from the above I think the biggest factor in choosing what style pet portrait to commission is what you LOVE the best and if you really can't decide have both like my client did! I know whatever you decide to go with, you won't be disappointed.
I hope that has helped a little bit, perhaps not, but please feel free to drop me an email and ask, I am delighted to help in anyway I can!