You can increase the dpi of your fonts to make them bigger on the Ultrasharp screen. I did that with my D800 to make it a little bigger so I didn't have to wear my glasses to see the screen.
To answer your main question, the internals in the Latitude are mostly the same as the inspiron. Every now and then there may be a small difference, mainly in networking cards/equipment. The Latitude is also tested to work with a wide range of networking equipment and be compatible in a business environment. The inspiron is geared towards consumer/personal use.
I prefer my Latitude D800 for build quality over my Inspiron 600m. Although the D800 is bigger it is sturdier than the 600m was. It does cost more, but I didn't pay for it. Just configure both of them and if they are similar in price, get the Latitude. You may be able to get a better video card on the Inspiron, and with certain Dell deals that come up from time to time you can probably get a really good deal.
Unless you need a big screen or you want more of a gaming machine, I would recommend the D600/600m. It is a little thinner and lighter than the 800/8600 if that's what you're looking for. You can get GREAT deals on the Inspiron 600m if you check xpbargains or fatwallet or even the Anandtech hot deals forum.