The pictures of Cambria hotel look nice, but I have no experience there so keep that in mind.
My answer depends on several factors. First, how many people in your party? Kids/ages? Touring style?
Second, don't underestimate the convenience of walking back and forth to your hotel. That location would definitely discourage going back to the hotel for a nap, rest, swim, etc. Even if it's all adults, that's a considerable amount of walking/waiting for a shuttle or uber, not to mention the added cost. You'll obviously be doing a lot of walking at DLR so you don't want to add too much to that.
Are you rope droppers? Do you take midday breaks? Do you plan to access the amenities like pool, play areas, etc?
My *personal* view is that I'd much rather stay as close to DLR entrance as possible. I realize that $500 is a chunk of money, but how often (if ever) will you be back?
How many days are you staying? When is your trip? There may be better options and prices for your dates. There are lots of good hotels/motels closer to DLR that are fairly reasonably priced.
My short answer is that I personally would find somewhere closer to the crosswalk to DLR, preferably right across the street on Harbor. I've not heard great things about ART.
If you're looking for a way to save money, maybe look at the Anaheim Majestic Hotel. It has a free dedicated shuttle and it's old but it is a full service hotel and tends to be very reasonably priced.