Personally, I created my own oracle deck first. Afterward, I worked on a tarot deck for a while, dropped that project for a while (it will be finished eventually, since it's fun and easy), completely rehauled and completed the oracle deck, and am now working on a serious full-sized tarot deck.
Not that my example is typical. In my experience, most people make one or the other -- an exception, off the top of my head, is Chronata of All Hallows Tarot fame. She's made several oracles and tarots.
I'd say it's a personal thing, which to make first, if at all. I made an oracle before I attempted a tarot because that's what I wanted at the time -- I still use the rehauled version frequently, because in some ways it's more personal and accurate than a deck someone else has created. It filled a need. Same with the tarot deck I'm working on now; it's designed to fill a need, albeit a different one.
I'll be interested to see the answers you get here!
