There’s a few advantages and disadvantages to each.
The main advantage with a head is that you can connect them to different cabinets to get different sounds. The guitar cabinet (and speakers) has quite a big impact on the sound, so some people like the ability to swap out the cabinet to get different tonal options.
(Of course, these days with modern digital modelling you also have the option of impulse responses which can pretty faithfully give you those cabinet tones).
The other main advantage is that they are smaller and lighter, so instead of one really heavy piece of kit to carry, you have two slightly lighter pieces of kit.
It also means if you are going to a venue where they are supplying cabinets, you only need to take the amp head with you.
I have done this with my Orange Micro Terror head. At home it’s plugged into a small 1x8 cabinet which is fine for home use.
I have taken it to a rehearsal room and plugged it into a 4x10 cabinet and it sounded very different!
The main disadvantage is buying an amp head and cabinet is they are more expensive than an equivalent combo, and it is then two bits of kit to carry around so, although they may be lighter, that could be an extra trip to and from the car.
Why are you looking at 100W amps, unless you’re going to gig there’s absolutely no point at all going for one that big! I would suggest as a start an Orange crush 35 RT, it’s an excellent amp plenty loud enough and it has a FX loop that is useful when you go into FX and a looper. For the price it’s one of the best sounding amps for rock and metal and sounds good for clean and blues.
If I didn’t already have an amp that I’m happy with I would definitely get that amp!