Choosing between the sled and a GHR all depends on what your goals are and what muscles you are trying to target. You said that you wanted to work on your core. Now the GHR is going to target your posterior. With the sled, you can get very creative on what you do with it and what muscles you want to target. There may be more carry-over to your core (not posterior work) when you are using the sled because of your whole body being involved in pulling it especially if you are dragging it behind you which targets the posterior. Dragging the sled backwards while facing it will target the quads more.
I have a sled and a reverse-hyper. I bought a very good sled for $90 from New York Barbells. I use the sled now as a means for recovery/GPP rather than for building strength. With the reverse hyper I never got good carry-over to my hamstrings so if the hammies are what you want to target then go with the GHR. The reverse hyper will really hit the glutes and lower back. The reverse hyper is another great rehab/recovery tool as well.
Louie Simmons created the reverse-hyper with back healing in mind. It really opens up the sacrum and gets blood in there. You don’t bend your legs with the reverse-hyper so I think that’s why I never really felt it in my hamstrings.
There are a lot of other great exercises you could do to hit the core and the posterior if you didn’t have a sled, GHR or a reverse-hyper. It’s great to have variety if you have the space and the money to purchase more equipment.