Likely the ZX81 - external / replacement ROMs were more common for it (and for its predecessor, the ZX80) because there wasn’t enough ram for much software. Th

Likely the ZX81 - external / replacement ROMs were more common for it (and for its predecessor, the ZX80) because there wasn’t enough ram for much software. The ZX spectrum had 16K and 48K models (with the latter being significantly more popular from day 1 and the only one available a while later). most software depended on specific addresses in the built in ROM, the 64K address space was fully utilized AND there was enough RAM so software in ROM wasn’t popular.