Specialized Diverge - School me on sizing, gearing, mods, etc

I am interested in purchasing a new Specialized Diverge.
Looking at the Diverge A-1, Diverge E5, and the subcompact
Basically an aluminum frame Diverge.
Not sure of sizing. I am 5'10" with a 30" inseam. I test rode a 54
and it felt fine. One Spec shop recommends a 54 the other shop
says 56?
Waiting for the shop to get in a 56 to try.
I've been riding mountain bikes for years, so not sure of road bike
sizing.
What about gearing, some models have 32/48(11-32) sub compact 
has 32/48 (11-32), E5 has 34/46 (11-34)?
Was told by a shop hat 32/48 is better for climbing, but not for
long straights/roads.(winds out fast).
I have seen a few you tube videos where people run a larger cassette?
Any suggested mods anyone has done to their bike would be great.
I know sizing is a personal thing, but any input/feedback would also
be appreciated.
thanks

i'm 5'9 with a 30" inseam and a 54 is a bit too short in the frame.  I fit a 56 Roubaix, Sequoia, and Diverge.  My guess is that a 56 will do you proper as well

48/32 - 11/32 is pretty good for a gravel bike.  You'll find that you won't wind out on gravel nearly as often as a roadie will on smooth tarmac roads.  The largest cassettes are typically on road-only bikes, and a lot of folks (like me) who have road bikes and aren't the top-notch racers will find a compact (50/34) or subcompact (48/32) a lot more useful than a full 53/39.  

What about gearing, some models have 32/48(11-32) sub compact 
has 32/48 (11-32), E5 has 34/46 (11-34)?  

My 2cents on gearing , I agree with sgtrobo about the 32/48  11/32 combo. 
My ride (a Raleigh Willard) came stock with a 34/46 11/32 and it has proven to be all I need where I ride. I am rarely in the 32 but it is nice to have when off roading or the occasional hilly climbs.
I think having a 32/48 crank would be sweet with the 11-32 would be my choice given an option.
With the 40c  tire's I usually ride I am rarely in the 11 t cog so winding out that gear doesnt happen and I can't sustain 30 mph for long even with skinny tires in the 11 t.  I guess it depends on your riding style.

Before you plunk any money on a bike you need to make sure that it is within certain parameters. Having done bike fits for over 30 years, I can tell you it is much easier to do a final fit for a person that has gotten close to getting the correct size. I sensed in your question that the bike shop(s) didn't really do this and I know it may come as a surprise but often the correct size just happens to be the one they have in stock.

If you can't find someone to do a fit for you I would suggest using the following site to do some self analysis. While I did Fit Kit sizing for all this time, this site's method is very, very close to it.

http://bikedynamics.co.uk/965ll.htm

Have someone help take the measurements for you and plug the numbers in on the site and you will get some very close calculations. Then go to Specialized's site and look at the dimensions (sizing chart) for the Diverges. The most important measurement, again after fitting bikes for over 30 years, is the effective top tube measurement. You start to mess with the handling of a bike if you have to use a stem that is too long or too short to get you in the proper position on the bike. The worn out theory of "let's have you stand over the bike and see if you can lift the front wheel off the ground by an inch or so," was a ridiculous way to fit a bike then as it is now. 

On the Diverge itself, great bike (started with an aluminum and now have a carbon). As far as the gearing, it all depends on your riding style, strength  and cadence. As a pro once told me, "Plan you gearing for making it up the hill, not coming down...as I never have trouble coming down."

Hope this helps.

Nice advice!  Definitely the effective top tube measurement is the primary fit criterion I use.

I found some good advice for gearing - but it is for fixed gear bicycles: find the gear where going down hill hurts as much as going uphill - LOL

Hadn't heard that one but love it. And yes, it would be so true.