Is there much difference between the orion apex 90mm telescope and the 105?

I'm buying a telescope and going to also mount my Nikon d300 on it from time to time. The difference in price between the two is $60 so I'm wondering if going from 3.5" to 4" is going to increase the light gathering power to warrant the extra money.

I would recommend that you consider the 127mm version, as I find the others have too small an aperture to be satisfying after a few nights. I would also ask you to consider how you are going to mount the telescope. Even the 90mm is a powerful telescope and will require a large solid mounting.

Orion has a good product and I have one of their 8" Dobs that I'm very happy with, but you're not going to get the views they are advertising on their site with that 3.5" or the 4" (other than maybe the Moon). They're showing shots of Jupiter on a level of detail that I would see through my 8" telescope on a good night with a high powered eyepiece. What they're showing is what a professional astrophotographer might be able to squeeze out of a tiny image in an eyepiece with a very very good camera and some serious photography skills.

If you really take those scopes to their limit on a good night, you're most likely to get a view more like this:

With a polarized filter (often called a moon filter) you could probably cut down on the light in that pic and just make out some of the weather bands of Jupiter.

Bottom line, the extra $60 probably won't make much a difference to you, but if you can swing it, it's always nice to have the extra aperture.

I would seriously consider a Dobsonian telescope of at least 6" of aperture if you want to get started in astrophotography. Although Dobs aren't really the type of scope that would be best for astrophotogrophy, they would suit you best right now because of your budget as they would provide a much more stable base. As stated, you're going to want a solid mount. That's what makes the price of most equatorial telescopes jump right up by hundreds of dollars compared to Dobs with the same size aperture. Unfortunately, that's also where they cut corners when they're priced about the same as Dobs.

Absolute last thing you want when trying to snap pictures is a bargain-bin tripod that wobbles and shakes constantly.

Go with the 105.
Hopefully, you have a decent enough mount that can handle the scope and the camera. Along with tracking and tracking error control.