When comparing Planck vs Let's Split, the Slant community recommends Planck for most people. In the question“What are the best mechanical keyboards?” Planck is ranked 17th while Let's Split is ranked 23rd. The most important reason people chose Planck is:
You can throw this thing in your backpack. You can take it with you everywhere. If you don't have a lot of space on your desk, this will probably fit. Don't confuse this for a netbook keyboard though. All the keys are the full size and are as far apart as a normal, regular keyboard. But there are fewer keys. The keys you can't access directly can be found via the "raise" and "lower" keys.
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Pros

Pro Tiny layout
You can throw this thing in your backpack. You can take it with you everywhere. If you don't have a lot of space on your desk, this will probably fit.
Don't confuse this for a netbook keyboard though. All the keys are the full size and are as far apart as a normal, regular keyboard. But there are fewer keys. The keys you can't access directly can be found via the "raise" and "lower" keys.

Pro DIY
You get to pick your switches. You get to choose your key caps. You get to select your case. You get to pick your layout. You get to reprogram your board. You even get to pick your firmware! This keyboard will be really and truly yours!

Pro Ortholinear key layout
This layout is different from a regular staggered layout. Some people really like this change in layout.

Pro DIY
You can really make this board your own. The seperated space bar makes it even more fun than the regular Planck because you can program both space bars differently. You can also get the keys much more easily because you just need a pair of single key width keys.

Pro Split layout
You can set this board up in a very ergonomic manner
Cons

Con Tiny layout
You have to build this thing. The keys are laid out weird. And now you don't even get numeric keys? WHERE DOES THE MADNESS END!?

Con Ortholinear key layout
You just went through all the work to solder this together and now you need to relearn how to type? Where does it all end?!

Con DIY
You have to pick your switches. You have to choose your key caps. You have to select your case. And then you have to build it.

Con DIY
Most people just want to buy a keyboard so that they can have it and type on it. But if you're ordering an ortholinear 40% split keyboard, you've probably already got strong enough opinions on switches, key layouts, and keycaps, that you're not going to be very satisfied with a mass market keyboard anyway.

Con Somehow cheaper to spec out than a Planck
This keyboard is based on the Planck. You would think that the Planck, what with its popularity, would cost more. Somehow, it's actually cheaper. This actually has to do with the case/frame. Instead of ordering a really nice prefabricated case, you end up ordering laser cut panels and spacers to create a frame to hold the whole thing high enough off your desk to type comfortably.
