Assuming the separates and combo use the exact some speaker and amp design, you are unlikely to hear any difference. Since you are not comparing apples to apple, you should expect the two options to sound different. Which sounds best depends largely on your taste and expectations. You might like the combo best and I may think the separates sound way better.
Small combos can be a bit more convenient, because you only have to grab the amp and power cable. With separates you need to grab amp, cab, power cable, speaker cable.
I generally prefer separates because it allows me to choose whatever amp and speaker I like. Each model of amp has it's own personality and characteristics. Each speaker has it's own personality and characteristics. In the majority of cases, I think the speaker tends to make more of a difference than the amp, but this is not always true. Some combinations can provide an unexpected gestalt, while others sound bad.
I do own one older combo of decent quality. It's a GK 700RB/112 combo. I chose it because it was the best amp (within budget) I could find that would fit in the trunk of my Dodge Neon. I would have preferred the 115 version but it was too big. The 210 version also fit in the trunk, but I thought the 112 sounded better. However, I have never been complete thrilled with the sound of this amp on the floor as the lows tend to be pretty loose and woolly. Elevating the cab helps considerably.
My last touring rig typically used a rack mixer and QSC power amp pushing an Eden D210XLT. IMHO this rig sounded way better than the little GK combo and it also could play considerably louder. It worked great for tours because it was packed in a rolling rack and loaded into a cargo truck. For local gigs where I would typically travel by myself while transporting my gear in compact car, the little GK was a better choice.
IMHO the best thing you can do is try to play a bunch of different amps and cabs. Keep note of the combinations you like and also consider size, weight, cost, and if they play loud enough for your needs.
Keep in mind, volume is not dependent solely on how much power the amp can make. AI made some relatively powerful amps, but their speaker were extremely inefficient. It think max SPL of their most potent combination was only rated for 118dB, which is not quite a enough for most cover bands, although works fine in quieter settings.
Years ago, one of my peers bought the original acoustic image combo amp with a matching extension speaker. The intent was to use this rig in big band. The cabs would be place on opposite sides of the band to allow everyone to hear the bass better.
The amp sounded great and was super portable, but it was not quite loud enough for big band. It worked great for jazz combo though as it was very small and light. A nice grab and go option, rather than dealing with a big amp.
