It's honestly hard to say which would be the priority, sub vs surrounds, since they are both integral to enjoying your system to the fullest. If you can afford it, the EP350(factory outlet) would be my choice over the 175. I think it'll give much more "presence" than the 175, as it should.

The Master and Comander cannon salvo scene is a great example as to why you need both surrounds and a decent sub. This part of the movie without either(sub/surrounds) just wouldn't be the same.

Half of communication is listening. You can't listen with your mouth.