Looking for next step lens

Hi

I've been doing some research for getting the next lens for my Canon 1100D.

The main use for this will be outdoor photography, nature, mountains, landscapes, astrophotography, and so.

I've been reading good reviews about 55-250, but lot of people suggest the 70-300 IS USM (in case budget is not a problem).

1) Is the 70-300 IS USM the correct lens for upgrading my actual 18-55? (knowing the use I'd give to them).

2) If you had both Canon 70-300 and Tamron 70-300 (which also read lots of good reviews), which one would you choose and why?

Thanks a lot for reading, and helping