I have stayed in all 3 hotels - #1
Disneyland Hotel (40, maybe 50 plus times), #2 GCH (3 times), #3 Paradise Pier (6 times, 3 of those booked by family and not by my choice). While we have stayed at GCH a few times and agree with others that it is beautiful, my family absolutely LOVES the Disneyland Hotel. For the same or slightly more than the price of a standard room at GCH, we get a concierge room at DLH. The concierge service is incredible and the staff that works in the lounge goes out of their way to make sure you are taken care of. We like to go to the lounge to watch the fireworks while having dessert on the day we arrive. It's a great way to start our vacation before we are able to actually head into the parks the next day. Our daughter is 11 (almost 12) and she loves DLH, as well. She still enjoys the waterslides at the pool (heck, I even enjoy them), the roaming characters in the lobby, and the headboards in the rooms. I especially love the nostalgic lobby, but I also love mid-century modern decor and have been staying at DLH since I was a toddler.
The first time we stayed at GCH, we had a horrible experience. Check-in took over an hour, we were put in a room that felt like it was the equivalent to walking from Paradise Pier to get to the parks, and the service was slow and lacked that Disney spark. We booked a cabana by the pool and the cabana attendant showed us to our cabana and then did not reappear again until lunchtime. It was incredibly frustrating and disappointing. We decided to write it off as fluke, but GCH never really rose above an average hotel experience the next two times we went.
That being said, the GCH lobby is beautiful and we will often sit in the lobby to listen to the piano player while our daughter has a snack from the quick service place near the pool. It's a nice place to take a break from the parks for an hour. And if it's Christmas time, you're in for a real treat. The lobby is even more beautiful! Christmas time at DLH is also fun. They have a HUGE gingerbread house in the lobby and Santa visits at least once a day. As a matter of fact, it's our favorite place to visit Santa because it seems that no one knows he is there. Last year, our daughter hung out with him for a good 15 minutes before another family showed up. The same with the roaming characters - a lot of people will just walk right past them and not interact with them. Our daughter once spent almost a half hour with Goofy pranking people in the lobby. It's an odd difference from the park where people immediately start forming a line to get an autograph with them.
The bottom line is - both GCH and DLH are clean and comfortable hotels to stay in. It really depends on how much time you plan to spend in the hotel and your personal preferences when it comes to style, distance from the parks, and what you consider essential in a hotel stay.