The larger the tiles the less grout you will have to keep clean and mildew-free. Your space is large enough to handle the larger format tiles. As you said, the rest is personal preference.

We've had both--6x6 with a decorative 2x8 decorative listello 1/2 way up in one bath in the last house and 10x13s set vertically in the other bath with the top row of tile having a raised design. We are building our last house and we've gone a different direction somewhat due to things we learned with the previous house. Here's how I see it:
6x6 pros-Potentially more design opportunities with laying the tile at different angles, it's a very common design, and you are truly getting every penny of your $6.50 per square foot install fee out of your tile guy.

6x6 cons-LOTS of grout lines to clean (better than 3x3s but still alot of grout), smaller tiles can make the shower look smaller visually, it's a very common design & it takes longer to complete.

10x14 pros-Fewer grout lines to clean, can make the shower look more open/larger visually, it's different and may look a little higher end, & quicker completion than a 6x6 design.

10x14 cons-Fewer design opportunities due to the tile being oblong in shape--do you want it vertical or horizontal, there is no in between & it's different.

You could always go with a mix of sizes to add visual interest--say large on the bottom 1/2 and small on the upper half with a decorative listello or chair rail piece placed in between or even just setting the 6x6s on an angle in between. Ours is just completed--our master shower is slightly bigger than yours at 48x94. (Note the listello got placed about 1-2 large tiles too low--I wanted it higher to help avoid the subway tiles getting wet to cut down on cleaning, but I didn't want to delay the job by having them rip it out and redo it so it stayed like this.) Looks pricey, but actually came in significantly underbudget (budget was $10sqft installed). Would I have been thrilled with all 10x14s on the wall like is in the 2nd picture below--with that particular tile design & size I do believe the answer is yes. Would I have been as thrilled with all 6x6s, I don't think so. In the end it all comes down to personal preference. Do you have a design in mind?

Here's a visual for you from our master-

Here's a tub/shower combo using all 10x14s in a brick pattern-

Here is a link that might be useful: 6x6 tile thread

I had the exact same setup (down to floor tile sizes and all!) and dilemma. I ended up, rather last-minute, going with the 10x14s in the shower, set on a horizontal brick pattern. Seriously, I love it. I'm so glad I did it, it looks so good and really makes the shower feel more open. I think I would have been disappointed in the 6x6s because that seems to be just a tiny bit outdated, and somewhat common and boring looking. Also, as everyone else says - less grout lines! That is a huge plus!