Every workshop is different, so rather than relying on general advice, it's worth getting your own set-up properly assessed. A system that works well for one shop can be quite wrong for another with different ventilation, floor space or process mix, and an on-torch extraction option, such as those available for Fronius torches, suits a different layout to a fixed overhead hood.
If you're still working out which process fits your workload, it helps to talk it through with people who field these questions every day rather than guess from a spec sheet, and the team behind welding tables UK in Thirsk deal with exactly this kind of query on the advice line.
Cutting capacity is usually described in terms of clean cut and maximum cut thickness, and the two are worth distinguishing. Clean cut is the thickness a machine handles with a good edge finish and reasonable speed, while maximum cut is the thickest material the machine will get through at all, usually slower and with a rougher edge. Buying with your typical material thickness in mind, rather than the thickest job you might occasionally face, generally gives a better day-to-day result.
Ambient temperature and airflow around the machine also affect real-world performance. A welder working in a hot, poorly ventilated space, or one that's been boxed in against a wall with no clearance for http://lotlab.net/index.php/If_You're_Not_Sure_Which_Supply_A_Particular_Machine_Needs_Or_Whether_Your_Workshop_Can_Support_It,_It's_A_Sensible_Question_To_Put_To_A_Supplier_Before_Ordering,_And_The_Team_At_TEC_Products_Can_Talk_Through_The_Options_Against_Your_Existing_Set-up. its cooling fan, will hit thermal cut-out sooner than the same machine used with proper clearance in a cooler environment. Keeping vents clear and giving the unit room to breathe protects both the duty cycle you paid for and the components inside.
Portability and power supply matter as much as the process itself. A stick welder will run from a generator or a domestic supply in places a gas bottle can't easily follow, while MIG and TIG set-ups need a gas cylinder and, for anything beyond light-gauge work, a heavier electrical supply. Workshop layout, the materials you weld most often, and how frequently the machine needs to travel are all worth weighing up before settling on one process.
Space and power supply are the other two practical constraints worth checking early. Confirming that a chosen machine will run comfortably from the electrical supply actually available in the workshop, and that there's room to work safely around it with materials laid out, avoids the common mistake of buying a machine that then can't be used the way it was intended.