You can certainly make do with only one sliding bevel, but in some larger projects you may be laying out several different angles throughout, and it is beneficial to set a separate sliding bevel to each angle and not alter them for the duration of the project.
When buying a sliding bevel, make sure the locking system works properly, make sure it locks down tight enough to stop the stock and beam from moving. The tightening mechanism can vary depending on the make, some have wingnuts which can protrude out and get in the way during layout. Some are tightened using a simple setscrew which lies flush with the surface, however this does mean you need a screwdriver to hand which can be a nuisance. The main advantage though is the guaranteed security this gives to the sliding bevel. Other mechanisms have cam lock-downs which also work well generally.
The Bacho ones seem to be very disappointing in the way that they do not hold the beam tightly meaning that they move during use and therefore cannot guarantee retaining the exact same angle. This is due to the levers not tightening as much as they should so they leave the beam marginally loose. This was tested on both types of fastenings. Paul recommends considering the modern makers when buying a sliding bevel as the fastening techniques might have improved so it is worth trying these. He suggests getting one with a lever cam or a lever bar as this gives you extra leverage. He found the Stanley sliding bevel has worked well for decades and you can get this new for £15.
🇬🇧We purchased this Stanley Tools 5025 Sliding Bevel 230mm (9") from eBay for £14.98* with free delivery. 🇺🇸If you are ordering from the US, we recommend this one from Amazon. Please note Paul has not purchased this, however it seems to fit his recommended criteria.
*Prices correct as of January 2018