Amp, cabinet, guitar, stompbox selection all depends on the sound you want.
Marshall OD tends to be more midrange than a classic Fender and the clean sound isn't as sharp. Fender is great for a tight, trebly crunch or a crisp, clean sound and the spring reverb is superb. Since I play classic rock cover tunes and most of them from the late 60's and early 70's used either Fender or Marshall, I have both. I have a Marshall JMP-1 with a Valvestate 8008 power amp into a Marshall 1960A cab. I also have a Fender Hot Rod Deluxe which I use for Clean and slightly overdriven sounds. The pair give me tremendous control over my tone from song to song.
I also have a Marshall 9200 Power amp which used to drive my rack rig, but i hurt my back a year ago and now it's too heavy to use so I replaced it with the Valvestate. Tone obviously isn't quite as warm, but with the tube preamp, still sounds good.
But it all boils down to what you like. Besides Marshall and Fender there are a lot of other good tube brands out there..Randall, some Peaveys, Traynor, Laney, Matchless. Each amp has it's own characteristic tone. You can find some old Peavey Classic 50's for about $150 and they sound good, as long as it's the tube ones. Randall heads are pretty reasonable now too.
And size really doesn't matter. If you've ever seen Brian May, he uses 2 Vox AC-30's mic'd. That's it. the others are back up.
Truth is that most tube power circuits sound pretty much the same, if they use the same tubes. Of course, things like bias and voltages can affect the level at which they saturate, but it's really the preamp section that provides the character of the amp. But as Jorma said, tube amp wattage is half what solid state wattage is. So for most situations 50 tube watts is sufficient, unless you're playing outdoors a lot or big venues. Even then, miking it works also. An old Marshall 50 watt half stack is still a loud, powerful amp.
I used to have a Peavey XXX with EL34's in it running into my 1960A cab, and on the OD1 channel you couldn't distinguish it from a Marshall at all.
And on the OD2 channel maxed out, it sounded just like a Triple Rec.
That was a very versatile head. But it was the preamp that allowed it to emulate the sound of other amps.
But so many different things affect your tone. A Marshall 1960A cab comes equipped with 4- Celestion GT12T-75's. They have a distinctive sound. But if you swap them out with Celestion Greenbacks, your overall tone will change. So even speaker choice affects your tone a lot.
The one thing I would say is avoid modeling amps like the plague. They don't really sound like the amps they're trying to emulate, except running direct. But, if you have to use a solid state amp, get a OD pedal with a tube in it to simulate tube overdrive. and only play the amp clean.
The biggest drawback to tube amps is the weight. They can be backbreakers.
