For me, I sold my 28-135 IS after trying the Tamron 28-75 f/2.8. The low-light possibilities of the f/2.8 lens overcame any advantage the IS had in the 28-135. Well, ok, the color, sharpness, and IQ were better as well...but that's all!
Then I started missing the wide end and now own a Tamron 17-50 f/2.8. It doesn't come off my camera except to put on the Canon 70-200 f/4L. I figured I'd use feet zoom for the missing 25mm. Having been to Disney 3 times within the past year and extensive testing between the 28-75 and 17-50, the 28-75 is now being sold to a friend. I'm picking up a friend's used 85 f/1.8 for the same price I'm getting the 28-75 for is it's an even trade.
That's just an example of how addiciting this hobby can get. Depending on how much you shoot, you'll find yourself going through a few lenses to find the best one. That's why I recommend trying the used lenses route first. Pricing is usually a bit cheaper and if its not what you want, you can sell it for the same price w/o the cost of depreciation from buying new. Just a thought. Everyone has different shooting styles. Most folks here woud never sell their Tamron 28-75 f/2.8 so to each their own
Based on your post, you have the Canon 18-55mm non-IS and the Canon 75-300mm. That will probably be fine for Disney. You could get a 50 f/1.8 for about $60-$70 used if you really want low-light performance. Or just go for one of the f/2.8 zoom lenses we posted earlier. It all depends on how much $$ you have and what type of pictures you are trying to shoot.