Timothy hay is generally just timothy grass, and it's often more consistent in its quality - hence why it's generally a little more expensive here in the UK. All the timothy hay I've bought was usually green and great quality (aside from petsathome hay, but they don't have anything good for buns). Now, meadow hay is usually easiest to find here (the only type of hay you can find at Morrisons for example) and is the cheaper option of the two. Meadow hay, in my experience with it, has been a little inconsistent in quality - sometimes it would be great, sometimes it would be stringy. And unlike timothy hay, it's often a mix of soft grasses rather than one of the same grass (like timothy).
My rabbits personally favour timothy hay over the two but will eat both just fine - and in the end, as long as they're eating enough of either, they would probably be in good health.
So both would be a fine option, there is no right or wrong answer to be honest. Whatever works best for you! If you are having a hard time deciding, just factor in things like budget, allergies, and of course, your rabbits opinion.