Solid-state vs. tube amplifiers for guitar
I don't think there's any such thing as a defensible proof that tube amps are better than SS, or vice versa. It's really a matter of taste and objectives.
SS amps tend to fill a different niche than tube amps. You'll have a difficult time getting a pristine clean sound from a tube amp.
OTOH, tube amps are revered for their distortion characteristics. It comes naturally with the design. A lot of discussion surrounds the character of distortion one gets from tube amps; this varies from design to design and even from instance to instance of the same design. Even tube amps have their sub-niches, as no one design serves all sonic goals equally.
Distortion has historically been an afterthought in SS amps, resulting in a lot of half-baked, awful-sounding circuits (e.g. the distortion mode on the JC-120). Recently some designers have put significant effort into emulating tube-amp distortion characteristics using SS circuitry. Consider the Peavy Transtube Series, the Tech21 amps, the ill-fated (now being closed out by the manufacturer at 20% of the original price) First-Act amps, and the boutique offerings from Pritchard (USA) and Bluetone (UK). I've played most of these (with the exception of the Pritchard and Bluetone). They're all different (as are tube amps of different origin) but all quite useable for a wide range of clean and distorted guitar tones.
One thing to keep in mind is that when you buy a tube amplifier you're not just committing to the sale price. You will also need to spend money to maintain and repair your amplifier. Maintenance is an ongoing obligation for owners of tube amps. The tubes themselves are the most common point of failure in a tube amp. They can degrade slowly with use. They can develop various undesirable noises. They can suddenly fail (much more common for power tubes than preamp tubes). As often as not, a failure will occur at a most inconvenient time (i.e. during a gig or session).
If you're prepared (by having stocked tested spare tubes and fuses and knowing how to find and replace the bad tube) and working with an older amp (newer amps sold in the EU conform to regulations that make tube access very inconvenient), then you can be up and running again in a matter of minutes. Otherwise you'll find yourself on a first-name basis with an amp tech.
Other than tube failures, a well-built tube amplifier should last a long time. However, lower-cost products of any type tend to be more prone to failure and more difficult to repair. Tube amplifiers are no exception. The higher voltages and temperatures found in operating tube gear tend to make failures more frequent and more catastrophic.
When a SS amp fails, there's no way that you'll be able to repair it on stage. That's the bad news. The good news is that failures are extremely rare, especially if you're using a SS amp designed for professional use (as opposed to a "practice" or "beginner" amp built to a low price-point).