Ceide Fields has a tour, musuem, and coffee shop. Carrowmore just has a guide, and I believe he is not onsite year round. If you go, I would call ahead to see if one is available. It is a very plain looking site from the road, and you could easily drive right by it. I think the guide made all the difference !
Carrowmore is fine if you know the history, and you know what you are looking at. I had no idea what a dolman was before our visit. I doubt I would have gotten much of anything out of just wondering around.
Ceide Fields was more tourist oriented, and was very well organized. We took the tour, looked arouind the museum, and had a nice bowl of soup and brown bread for lunch.
Ceide Fields in very cold and windy, dress warm !
I have some pictures of both, and if you want to private message me, I will send you an invite to my Snapfish account !
Also consider Carrowkeel, a much less visited place and considered by many to be more impressive. It's WAY off the beaten track and you may not see anyone else there. The tombs are big enough to climb inside and the views across the valley to Lough Allen are impressive. I talk about it on my 2005 web page here:
You have received great advice already from many forum members. I agree with them all. Let us know what you decide on and tell us about it when you return.