We have been to Magic Mountain three times in the past three years. Our first trip, we stayed at the Comfort Inn in Castaic. I would not recommend that, as it is in a questionable area. The next trip, we stayed at the Hilton Garden Inn. It was fine - no complaints. For our most recent trip, I noted that the Holiday Inn Express was less per night, included breakfast, and was higher rated on TripAdviser. Against the wishes of my wife and kids, who were quite happy at the Hilton, we stayed at the Holiday Inn Express.
The Holiday Inn Express is almost as close to Magic Mountain as the Hilton. It is on the east side of the freeway (I-5). The driving time to / from Magic Mountain is about the same, maybe a minute longer.
The Holiday Inn Express was built in 2002, and still seems very new. The hotel has few frills, though in room high speed internet is free, and a refrigerator and microwave are standard. We stayed in what was called a suite, though it really wasn't much bigger than a standard room. It did, however, have a fold out couch-bed in addition to the 2 queen-size beds.
The Holiday Inn's strengths were how clean and quiet everything was. We were a bit nervous when we pulled in, as the parking lot offers an excellent soundtrack to the freeway noise from I-5. Once the hotel doors shut behind us, however, it was remarkably quiet. This was true even in our second floor room - a room which faced the freeway, giving us views of some of the larger Magic Mountain rides.
I would definitely return to this hotel over the Hilton Garden Inn. Combine the lower price with the slightly larger room (if you get the suite) and the free breakfast, and the Holiday Inn Express Valencia is a great value.