The 634 has a nice large character size (7.3mm) compared to the "standard" character size that others use (5.55mm or smaller, typically). This is generally good since it allows you to read the display from farther away. It can be a draw back if you are working with very tight mechanical constraints. The 632 and 634 both have small PCBs compared to their viewing area--there is not much "wasted space" around the active part of the display.
The 634 is also very thin compared to some other serial interface LCDs. Some of them use a separate "daughter board" to hold the additional electronics, which makes the complete assembly thicker. Compare a 634's 19.9mm thickness to a typical serial display's 27.5mm thickness. The single-board design also means that there are no board-to-board connectors, and if they are not there, they can't fail.
The 634's FSTN LCD is pretty hard to beat as far as display quality. More expensive than the run-of the-mill STN, but it gives a nice edge in optical quality.
The backlight on the 634 is pretty killer:
There are 70 die bonded LEDs in this custom beauty, and if you want brightness, you get brightness. Compare the 634's 500mA backlight current with other manufacturer's typical 90mA to 250mA backlight current. But what use is power without control? The 634's backlight can be dimmed from "just barely on" (which is good if the display is in a very dark setting, like a car dash at night) to full power. This dimming is done under software control and there is no additional heat generated by the dimming circuit. Some of the other manufacturers are offering EL backlighting. The color of an EL looks cool, but it is absolutely not as bright as an LED backlight. EL backlights also typically have a 3000 to 5000 hour "half-life" after which the backlight is only half as bright as it was originally. If you are planning on leaving your display on continuously, this half-life may be an issue.
Since the 634 uses a custom LCD glass we have the unique feature of being able do continuous horizontal bar graphs and the nifty scrolling marquee.
Another nice feature of the 634 is that the entire display state can be set as a power on default. Everything can be set just like you want it: The display data (OK others do that too), the cursor position, then cursor style, the contents of the "custom characters" (you need that for your logo), the scrolling marquee data and settings, the contrast, the backlight, the "wrap" setting, and the "scroll" setting. This feature is really useful in a PC, since the display wakes up in a few miliseconds, and the PC itself make take a couple of minutes to wake up. During that time, your custom "Loading OS" message can be happily scrolling across the LCD--fun, fun fun.
The cabling is quite simple if you are connecting a 634 to your PC. Just get a standard "db-9 female to db-9 female straight-through" cable, plug it in and you are done. The LCD section self-powers from most serial ports, which also allows the display's power to be turned on and off by the PC software. The backlight does require a separate +5v connection--we offer a low-cost backlight cable that makes it a snap. If you order the backlight cable with the display, we will solder the mating connector to the LCD so your job consists only of connecting cables.
Well, in the end you need to look at the specifications, look at your needs, and then decide which product will work for you.