The answer depends on a few things, your budget, your longer term plans, and your available space.
If you want to engrave metal, look for a fiber laser.
If you want to just engrave on wood a diode will work well.
If you are hoping to cut 3/4 inch boards a CO2 laser will be needed.
I've no experience with fiber lasers. I did research both CO2 and diode lasers.
If your shop is subject to freezing you will need to stay away from a CO2 because it needs a cooler.
Price (at the time I bought mine) and a shop which can freeze caused me to go with a diode.
I chose a Rolyautomation 10W and then upgraded to the 20/10W. Great service and a solid machine.
Look for these things which you will likely need if you do get a laser (in no specific order):
1. Air assist
2. An enclosure
3. Exhaust fan
4. Honeycomb bed
5. Rotary (roller style works well for pens, but a chuck rotary is more versatile)
6. Lightburn compatible (not an absolute must but real nice)
7. Built in camera
All of these are included with the Roly. You can get it without the rotary. It is possible to engrave a single line of text on a pen without a rotary.
Rolyautomation.com
Good luck!