The BennVenn(64) as it is available now provides a patched version of LSDJ with 128K of SRAM, allowing the use of the LSDJ file system (16-18 average length songs)
The BennVenn(256) as it is available now can support up to 3 patched LSDJ ROM's each with their own access to 128K SRAM, and the remaining ROM space can be packed with whatever other ROM's you want to store on the cart (each with 32K access for saves)
The cart Reader/Writer sold with the carts has the software to build multiple ROMs with a boot menu, all automatically. SRAM is managed automatically also with human intervention if/when needed.
As far as I know, the 256M cart has the largest Flash volume of any GB cart (Please correct if I'm wrong) allowing instant access to multiple ROM's and Saves without the need to transfer from USB or SD card.
The USB Linker supplements the carts by giving quick, reliable, convenient access to the .Sav's on the cart (Or any cart flashed with my patched LSDJ ROM) while in the gameboy.
A utility similar to LittleFM was being developed for my carts until the 128K patch was released. I really can't see the need to archive .Sav's to the carts Flash when a Linker can send them to your PC for reliable storage.
There may be situations when the USB64M or Derp have the edge... The Everdrive-GB supports Mappers 1,2,3+5 whereas the USB64M, Derp and Bennvenn only support MBC5. This becomes an inconvenience when playing certain games written for earlier mappers or that require an RTC and is not a problem with LSDJ or other Chip music ROM's.
Derp uses FRAM in place of USB64M, Everdrive and BennVenn's SRAM, so no battery is required and the possibility of data loss due to a flat battery is removed. The typical lifespan of a quality lithium coin cell in this application is anywhere from 3-10 years (My original GBCam battery is still good almost 20 years later!) The benefit of FRAM is for you to consider...
Once the ROM is booted, no cart can perform any faster or slower than another.
Other differences between carts include the ROM transfer method.
Drag-n-Derp and USB64M as the name suggests uses a USB cable to transfer the ROM to the cart
Everdrive uses an SD card which you fill with ROM's and insert into the cart. A boot menu then copies the ROM into Flash.
BennVenn carts use an external writer to transfer ROM's and SRAM. (Or by Linker cable*)
There are other carts out there, the older Bung/Mr Flash carts can sometimes be found on auction sites or forums though the hardware used to flash these carts is difficult to use on a modern PC and even harder to find (This is where the Altane is useful) . These carts are typically very small and contain only 32K SRAM.
I think this thread is a great idea, I'd like to learn more about the Nanoloop & kk_gb_8m hardware and whatever other carts are floating about