I found a LOT of gold with the 4500, way more than the 5000 but my kids have taken more of my time since I got the 5000 and I hardly get out anymore.
My 4500 had a more erratic and unstable threshold, but if you use it a lot and get to know it, it will sing to you! You get used to the unstable threshold. I had one of the first 4500's and always wondered if it was supposed to be like that, but dang I was finding a lot of gold.
The 5000 found a piece of gold in a spot that had been hammered by me with the 4500 many, many, many times. I found the piece with the same coil I used in the 4500. A 14" elliptical Coiltek. As you know many things can factor into why you hear a target now and didn't before, but I am 100% sure I found a target with the 5000 and the 4500 missed it. It was a 2+ ounce piece of leaf gold that I wrote about in the ICMJ several years back.
I didn't find a bunch of gold with the 5000 that I missed with the 4500 in patches I worked except the big nugget. If I had a choice, it would be the 5000 because it is more stable, but if you have a chance to get a steal on a 4500 take it.