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In my uneducated opinion, You are comparing Apples to Oranges.
The SideKick is a portable LP Grill. Much like what I carry around in my RV for "Camping"
But the SmokePro bbq sear box is an accessory for for a Campchef Pellet grill to sear your meat.
If you have a Campchef Pellet Grill, it looks like a perfect accessory to get those sear lines in your creations.
But if you want a portable LP grill, then the Sidekick (or many others) would be nice.
I get my grill lines without actually branding my meat, per sey.
Camp Chef's sidekick is attached to the smoker. It doesn't seem be be the same as the one on Ebay.
"The Sidekick is just that—your pellet grill’s powerful partner in cooking crime. The propane powered, single burner accessory easily attaches to most Camp Chef pellet grills. It's designed to give you VersaGrillity™ as one of Camp Chef's 14-inch cooking systems, meaning you can add a griddle, grill, artisan oven, stock pot, and more. The Sidekick has 30,000 BTU and is perfect for searing. Grilling now has more options and greater convenience with the Sidekick."
Yeah, entirely different. Looks like the Sidekick is Camp-chef's answer to a side burner. But better.
It has some interesting add on's to it. Like the Pizza Oven.
Where the Sear Box is more limited to being a searing add-on.
I think I would go for Sidekick as it seems to me to be more versatile.
But that may have to do with me using my side burner a lot.
I can get grill marks on my grill. But boiling my taters takes a side burner. ;)
Tough call. Do you think you would be more prone to enjoying having a side burner?
Here is another thought, you can do searing with the Sidekick and one of these type Lodge pans.
So you could have your cake, and eat it too. :D
I bought my Woodwind SG with the sear box. My Woodwind is only used at home so if I need to cook additional dishes I use the stove in the house. I also do sous vide meat cooks and the sear box is great for doing that final sear to my steaks and chops. If you use the grill in remote locations, then the sidekick might be the better choice. But at home, not so much.