There are 3 types of laptops.
Rugged, designed to run in extreme environments, Designed to run while be bounced around in a car, in the rain, cold. These are usally high dollars units and have a narrow market nitch. They are often not top of the line in speed memory or disk size.
Business class. This is what most laptops were a few years ago. These are designed to take rough use while turned off, bumping them through airports and dragging them around when you travel. These range in specs but usually have options such as docking stations needed in a corporate world.
Consumer class is the new class, really picked up the last few years. It has basicly replaced the entry level desktop. Prices vary from a few hundred dollars to several thousand dollars for a high end gaming laptop.
In the early day of computers slots and upgrade ability was an issue, You had a seperate video card, network card, i/o card and even an optional mouse card. but now most desktops and laptop's have pretty much all features/functions built in. The number of people who actually add additional cards to a machine is small, Heck Most motherboards now only support 1 or 2 I/o slots where 10 years ago 6 slots was not unusual. Now a days, laptop memory are standard. Not the same as desktops but within there class are standard. SO memory and disk upgrades can be made on either desktop or laptop.
Reliability is more on the manufacture and not weather its a desk/lap machine.
For most people a laptop is just as good as a desktop when comparing like cpu/memory class machines. I personally would only want a laptop as my only computer IF I had an large external monitor and keyboard for it, but with some of the larger 17" laptops you can almost get away from an external
monitor. I drag a laptop around at work and then have a docking station with 2 19" monitors on it. Also have a desktop at work with 4 monitors on it.
At home I have an older desktop but I have a number of special purpose I/O cards that need slots. I also an older large frame laptop I use if I use.
Most computer gear only has a 3-5 year functional life span so get what you need now and figure it will be replaced in a few years, support and repair issues pretty much go away.
I am about ready for a new machine and will most likely get a laptop for my needs. Ill keep the desktop for some specialized work but the laptop will be my primary box.
Nevada: yes there are reasons to keep 233mhz machines. I have some software that will not run on newer faster machines and have to go back to the old dos laptop to run.