At the end of the day they're both good e-readers but for me the larger size Forma was actually the big reason I switched. Portability isn't a factor for me but if it was I'd go for their smaller Libra model. I find the Forma lighting considerably better and more even and the font rendering better as well. I'm also not a big fan of the Oasis cold metallic feel... much prefer the Formo plastic / rubbery feel. I think for most it's a preference thing.
Kobo Forma. I'm an avid eBook reader so the little things add up. Going from the Kindle to the Forma was a huge upgrade. Font rendering, warm light, larger form factor, and Pocket integration amongst a host of other little niceties make it pretty much a perfect experience for me.