Cedar.

Unless the oak is white oak.

There are many different woods that are great for greenhouses but from the two choices you mentioned, either would be fine given you are going to neglect them anyway.

...and since it won't matter if either lasts very long, you may be surprised to hear that both of those you mentioned are still likely to last a long time. Oak as a hardwood will hold up well as will the softer cedar. Both will require maintenance of some form or fashion though so don't expect miracles through neglect.

Personally, I like cedar and prefer cedar in many projects but as small as some of the hardware is for your project, I would prefer the oak just because the screws will hold/stay in oak better than cedar but expect more serious maintenance and higher expenses down the road if you don't keep either choice maintained. Wood outside always needs help when it resides in the elements.