1. Their Faces have the Same Width
The number "30" indicates the width of the face of the bulb in 1/8 inches, so both are 3.75" wide. This allows them to fit into common 4" recessed lighting cans. They could also be used in 6" cans, but they'll leave more space to all sides. This is just a different look that you may or may not like. If you don't care for it, then you could move up to PAR38 or BR38 / BR40 bulbs instead.
2. The Main Difference is in Light Distribution
We consider BR lighting to be optimal for general lighting while PAR lighting is good for emphasizing certain areas or objects. PAR lights tend to put a harder edge to their lighting with more shadows outside their beam. So while BR lighting might provide more consistent light throughout a room, PAR lighting might give more emphasis to a sitting area (for instance) while more dimly lighting the rest of the room.
3. PAR30 Bulbs have Two Neck Options
PAR30 bulbs are commonly used in both recessed lighting fixtures and track lighting, so they come with short neck and long neck options to accommodate this usage. PAR30 short necks are more often used in track lighting while the long necks are often used in recessed lighting cans. BR30 bulbs do not offer this option.
4. PAR Lamps are More Often Used Outdoors
If you're going to use lights outside, the lamps (aka bulbs) need to be "wet rated" -- or they need to be used in a fully-sealed fixture that doesn't let moisture inside. (If the bulb is "damp rated" it can still be exposed to moisture in the air, but not direct water.) Some PAR30 lamps are wet rated; BR30 lamps are usually not. So if you need a wet rated bulb, check the packaging or online listing before buying to make sure, since there is no fixed standard about this. You can find our wet rated PAR30 LEDs here.
We hope this helps you make the right decision between these two types of lamps, and encourage you to visit Lighting Supply to place your order online, or to give us a call for help placing your order.