A rompler usually has keymapped, multisampled waveform patches or drum kits stored (burned) into flash rom. They are usually of low quality, thin, highly compacted, and tiny snippets of sound over-processed to make them sound decent. They tend to cram as much samples to make patches onto the smallest amount of ROM as they can get away with. Usually with these romplers, you have the effects processors defaulted to ON in order to make the thin and small samples used. Also, even though a keyboard or module claims to have over 1200+ or so patches, a great majority of these use different combinations of the same waveforms tweaked in some way by the engine to make them sound different (i.e. creating different patches).
A sampler is different. A sampler can sample external sounds and depending on the sampling engine, those samples can be manipulated in many ways. The number of samples is determined by the amount of RAM. Which can also be cleared off to make room for new samples (whereas ROM, what is burned is what you are stuck with...while you can create your own tweaks with the engine of the existing waveforms...you will always be stuck with those waveforms). A sampler can also act as a sound module much like a rompler. A few differences though. A sampler can only hold as many patches as the onboard RAM will allow for space. Usually, a rompler's patches are about 1200 stored into 64 MB of rom...whereas in a sampler you may only get like 10 patches or so loaded in the same amount of 64 MB of RAM. THe difference is that the sampler's files are bigger. And yes, bigger means better. The sampled keymapped patches use bigger samples that allow for more breadth of the sound...if you will. They do not rely on effects to create the full sound With sampler's the effects are applied after the patch...not with it by virtue of be defaulted to ON in order to make the full sound.
To put it another way, if you have a sampler that can import multisampled patches...a rompler is useless because it can never compete with the quality of the sampler counterparts.
I hope this made at least some sense.