The Sansa will read everything in internal and microSD memory together for Album, Artist, etc. It builds a database–an index–from the tags, the electronic labels in the files.
If you put the unit in MSC mode, which I recommend, it looks at the card slot and the internal memory as two separate drives if you are viewing it from the computer. So it’s your choice where the music goes. But to the unit it doesn’t matter once it’s in there.
I don’t know how different internal and microSD are for battery usage–probably very lttle–but speed is the same.
Settings/System Settings/USB Mode. MSC is a straightforward mode, like having two flash drives for the internal memory (4GB or 8GB) and the card. MTP lets you do stuff through Windows Media Player, which makes the unit a virtual device blah blah blah…don’t bother unless you are using library audiobooks that need MTP for their hidden unlock codes.
DO change to MSC rather than Auto Detect or you will probably get Auto shunted into MTP. The idea was to make Windows Media Player/Sansa work like iTunes/iPod but it’s not that easy. If you think about it, Apple controls both software and hardware, so iTunes only has to work with a relative handful of gizmos. Windows, however, is on all kinds of hardware with all kinds of quirks, so Windows Media Player is a messier thing trying to cope with them all.
If you are really attached to old ways, the unit also has a Folders setting (under Music) that lets you look separately at internal and the card slot as if they were separate, but you don’t have to.
It IS trickier to make playlists that bounce between internal and card-slot storage, but it can be done.
I keep all my music on a 32GB card and zero on internal. If I were you, I’d get the cheaper one and do that. So if the internal memory ever goes bad, you can just move all your music into the next unit by popping out the microSD card. I believe that you can now even use as 64GB card if your format it to FAT32.
I just bought a 64GB SanDisk Ultra for $28 on sale at B&H. Those sales keep coming around, memory keeps getting cheaper. All you really need is a microSD player, and the Clip+ is good at that.
Get the 4GB, set Settings/System Settings/USB Mode to MSC, put everything on the card, have fun.