I would think that the type of device (tube/transistor) used would be determined by the type of primary power you have available, AC or DC. If you are talking about an amplifier that has AC power available, say at home, then the high voltages required by tubes is easier/cheaper to make than the low voltage but high current required for transistors. the opposite is true for where DC is available, say in a vehicle. Then it's a matter of the power desired, a single tube is capable of producing much more power than a single transistor. (That depends on the tube and transistor of course.)
It makes more sense to me to use tubes when talking about an amplifier at home. It makes more sense to me to use transistors if there's no AC available.
'New' or 'old fashioned' has little to do with it other than for availability. What about heat production? So what, both produce heat, deal with it. What about the other 'bad' characteristics of tubes or transistors? Same answer, deal with them. Both devices have been around long enough that the 'how' of dealing with those characteristics are fairly common. Size? Tube type amplifiers are generally larger in size than transistor amplifiers. What's so unusual about that, make room for whichever you decide on. If you are limited to very small spaces to put an amplifier then a transistor type should do okay, but in a fixed location you still have to figure the room for a rather large power supply. Use which ever suits your situation the best/easiest.
- 'Doc