Desktop vaporizers are mainly for dry herb but you can also get sophisticated units for waxes and concentrates.  The main think about desktop units is the added power.  You also get more control with finer temperature settings.  Large desktop units are usually plugged into mains power and can produce a lot of vapor cloud compared to the smaller and more discreet mod or pen units. Some use a balloon bag to collect the vapor produced.  Desktop units are not designed to be easily moved around but you can expect lots of features on these units along with the ability to do very fussy temperature adjustments when producing vape cloud. If you want to make lots of cloud and want to do that with a few friends, then a desktop is a good pick, but expect to pay more for these units. Portable mods and pens rely on batteries needing recharges and less power, but battery technology is getting better and better in terms of power output and life between charges.  Mods or boxes are bulkier but offer better setting features, bigger battery power and usually make it easy to change a cartridge or load and unload dry herb. If you want a portable for around the house or on a camping trip, then this is probably your pick. Pens are very discreet but usually limited in battery life and settings.  They also usually hold much less material for vaping. If your main thing is discreet use at a party or out and about, then a pen is ideal. You will also find hybrid models that offer use with combinations of oils, concentrate and dry herb (not vaping these materials together, but you can use a hybrid device to swap from between each material, and usually you will need to switch a tank or chamber for this purpose). We personally prefer a device for each material, so if you like oil (vape juice) sometimes, then get a pen for that purpose. If you also like dry herb or wax, get a dab pen for wax and a nice mod for dry herb.