Would it be better to take public transportation and the RS book for a day to Avila or take a tour? If it is a tour, does anyone have any recommendations.
We did it with the RS guide book. Avila is a quick hit. We did it on a loop out of Madrid to Avila to Segovia (overnight) and back to Madrid.
Avila has a great wall. We also liked visiting the churches and St. Theresa museum (although we didn't see her finger). Many sights' hours include a break in the middle of the day. If I took a bus to Avila, I'd be concerned I'd miss a lot arriving at the middle of the day, then have to leave before things open back up. A tour will know the open times and make sure you get to see what you came for. We didn't take any tours but noticed the bus stations (which house multiple companies) also advertised tourist information. As far as I could tell the tourist information they offer is short bus tours.
Although commercial bus day-tours are unfashionable here, some years ago we found the Trapsatur (Madrid) company a very efficient way to get to three or more sights in a day, with no car rental, parking, driving, etc. The key is to find a tour that you like the content of, and make the most of it. For example, we made the "mistake" of taking the optional lunch option on one such tour. The food was fine, the price was fair, the restaurant was not unattractive, but we could have had 80 more "independent" minutes in Segovia if we had settled for a pre-made sandwich from a storefront while walking. The guides were so good that I was happy to tip them. Avila was a little rushed, but we saw the high points, and felt we got our money's worth.