Hi I am looking at buying a used Canon 5d Mk1. I currently have a Canon 50d I know there is a megapixel difference and I need some advice on if I should upgrade to the 5d mk1 or stick with the 50d? 50d is crop sensor but has more megapixels than the 5d MK1.
I wouldn't think too much about the pixel difference, you will probably not really notice it (if your not truly into testing of course). Here is a link that compare the two camera bodies: http://snapsort.com/compare/Canon_EOS_50D-vs-Canon_EOS_5D/detailed
However, the two bodies are getting old, the 5D mk1 is over 11 years old by now and they are getting outdated. The 5D mk2 is better than the two other bodies in every way and is now getting really cheap on the used market! Here is a link showing the differences between the MK1 and MK2: http://snapsort.com/compare/Canon_EOS_5D-vs-Canon_EOS_5D_Mark_II/detailed
I have had the 5D mk1 myself, and it's still good imo. But it's really up to what you are into photographing. If you like photographing portraits with a prime lens, it's still doing its work well. If your into sport photographing, or anything fast phased where you might need to handhold your camera a lot, maybe you should reconsider.
Megapixels In my opinion is not that significant, except in the case you really need them of course, but the jump needs to be double, for example from 12 to 24.
In my opinion, the best reason to upgrade a body is ISO sensitivity and noise. That difference can open you some new doors to use ambient light in a new way.
Changing from a smaller sensor to full frame could mean that you might not be able to use some of your current (EF-S/crop body) lenses, so take a look at that.
You don't just look at megapixels claim - you test the two cameras and make a decision. I have rather poor experience with a camera that has impressive 20 megapixels claim but the fact is if I open the image in Paint and zoom it to one hundred percent I see tons of noise everywhere and the image is barely usable and I have to downscale it four times each dimension (25 percent height and 25 percent width) which basically means the pixels count gets 16 times smaller and I actually get a 1,25 megapixels image. The 20 megapixels claim is worth nothing in this case.
So first look at the numbers, then test yourself, then make a decision.