Well, first off, they aren't all the same amp. There are differences between them, including wattage, number of speakers, and speaker size.
First off, cross the Marshall 15w off your list. Most 15w amps are practice amps, and are poorly made. The MG30 is probably the next worst in terms of tone... its not absolutely unusable, I think, not like the 15w MG is, but I don't really like the tone that much. It has too much gain, and not enough of it is usable before it turns into a fizzy crunchy mess. My opinion, at least.
I actually like the MG50 quite a bit. The speakerbox is a little oversized for the amp, which means a little more emphasis on the bass. When compared side by side at the same volume (not full blast, but at moderate practicing volumes) it has more low end "chunk" than the MG100 stack does, at least with the stock cab.... that was nice.
I haven't tried these particular 100w models, but I would be able to tell you that the 2 speaker model will "push more air", and will be heard better in a band context especially. Its not that its necessarily *louder*, just that it spreads the sound out more. Personally, I think 2x12's (whether in combo or head/cab form) are ideal for gigging and practice, as long as you have enough wattage to be heard when practicing with the band. 4x12's are heavier, bulkier, and while they might push more air and have a fuller tone, a lot of the time that = more mud than justified, I feel. A small cab has less of a low end emphasis, which is actually better when you're in a band situation or recording.
Anyways, I digress. The MG series gets a lot of unfair criticism from many in the guitar community. Part of it is because they're one of the few amps Marshall has made recently that's completely solid-state. Even though Marshall put out the Valvestate models years ago, they were billed as hybrid amps, since they had a tube in the preamp. Really, though, the tube was run starved-plate, and didn't contribute significantly to the tone of the amp - the amp was for all intents and purposes a solid-state amp. Beautiful sounding amp, though, I kinda regret selling mine, even though it was a little hissy.
Its very easy for guitarists to become tube snobs and claim that only tube amps sound good. Load of bunk, really. BB King got his tone from a solid-state amp, for instance! Quite frankly, I think that a number of recent Marshall tube amps kind of blow... another consideration is that you have to crank a tube amp for it to sound *really* good. The older Marshall tube amps may be held as archetypical tones (ie JCM800) but to sound that good you have to crank them, which means they end up too loud to be useful in any reasonable situation. Solid-state amps don't have that limitation!
I digress. More watts = more clean volume before the amp starts to break up and distort. Amps with more watts will tend to work well with effect pedals, and that goes for both tube or solid-state amps.
Lower watts = more distortion at lower volumes. If you don't want to be really loud and still get good tone, you want a lower watt model.
I haven't tried the 5w tube amp. I'd be interested to, but it would only be for playing by myself, not for playing in a band or anything like that. 5 watts, even with a tube amp, doesn't give you the volume to compete with a drum set, bass guitar, etc. More than loud enough to play in a bedroom or apartment, though, and loud enough to go through the walls and piss off your neighbors (ahem).
I don't look at effects when I play or try out amps. I don't like onboard effects, I don't use them, I think that they represent money that should have been spent making the amp sound better (ie better components, etc).
So it kinda depends on what you need. For home only playing, I'd at least try the 5w, if I didn't like it then I wouldn't get any of them - the others are too loud (50w, 100w) or don't sound good enough for me (15w, 30w). In a band scenario, I'd choose the 100w 2x12, since it can push the most air. 100w is more than enough to fit in to just about any band, as long as there isn't another guitarist with a 4x12 who just loves to turn it up too loud.
My buddy plays through an MG in a local band - their style is somewhere between Tool and System of a Down, albeit with much more intricate songs then even Tool. He sounds frickin' awesome - not "muddy" or any of that other junk - he can get anywhere from a great rock sound a la AC/DC to a very decent thrash metal tone a la Metallica. He's also using decent gear, though - guitars with EMGs or Blackouts, and a nice speaker cab I sold him (1960a 4x12, loaded with GT75-12R speakers, frickin' awesome speakers). The MG series tends to sound extremely good with decent pickups and decent speakers!
Saul