Interesting Amanda that you didn't like the Tamron 70-300 - although I guess it depends which of the 2 Tamron 70-300's you tried. If the cheaper LD Macro version, indeed attimes that can be a bit cumbersome compared to the later Di USD version, which is far better. The Tamron 18-270 is a good option, as is the Sony 70-300. Strangely I have all those lenses (the 18-270 I bought as a "fit and forget", the Tamron 70-300 LD Macro I got with my A700 secondhand, then bought the Tamron 70-300 Di USD to upgrade it (as it is a far better lens) and only used that a couple of times (with excellent results) before I bought my A77 and got a great deal including a Sony 70-300G lens).
Which do I prefer? Well I'll be honest, I do a fair amount of photography up at Colchester zoo, and fit either my 70-210 Beercan or 70-300G, depending on what it is I intend to take on the day.
The Tamron 18-270 I'll take as a walkabout lens if I'm taking my camera out for the day with no idea what I may come across. A very versatile lens I have some excellent shots with it.
You mention the Sigma 150-500 lens. I have that also and it is a smashing lens, but a little on the weightly side. To be honest I think it a little large for zoos, but it depends which zoo. If you are doing a distance shot, then it is excellent, although you may need a mono or tripod if carrying it all day.
Personally? I don't think you'll go far wrong with either the Tamron 70-300 Di USD lens or the Sony 70-300G, both give sharp results and excellent pictures.