To first get into the subject, here is some basic background information.
1) The first round, will be about value for money. If you have £300 pounds budget for an amp, and need an amp with enough power to compete with a drummer at a gig, a solid state amp has your back. You'll most likely get a channel switching, built-in effects and a good range of tones in a package that won't put a permanent curve in your spine, along with good headroom.
In the case of valves, £300 may buy you a fine five watt amp, (which is very loud for 5 watts) like the blackstar HT-5 which is what I have, or a Marshall class 5. You're not going to get a fire-breathing gig monster, but the amp will be easy to crank into harmonic distortion overdrive (will talk more about this later), and it is easily miked up if you have the extra gear.
Who wins this round? Up to you, and your budget!
2) The second round, will be about consistency and reliability. Solid state amps are often lighter, do not run as hot, and sound pretty much the same (tone wise) at any volume, apart from when it is cranked to the very max. Valves on the other hand, run hot, are heavier and the valves are vunerable to breaking if dropped. On saying this, I know many people who have had valve amps for over 10 years with the original valves still in. Myself? I have had my blackstar HT-5 for 3 years, with no faults. My solid state amps (line 6 and fender) have only had minot faults.
Who wins this round? I'd say it's a tie.
3) This round goes to pure tone! This is where valve lands the sucker punch. When valve amps distort into overdrive, they distort harmonically (twice) which gives the sound and tone a feel of natural warmth and crunch. Also the bass mid range frequences generally sound clearer always triumphs over the tinny sound from solid states. On saying this, there are plenty of decent solid states that still sound quite good, such as the Randalls, which are hybrid amps, which combine the technology of both, using "mosfet" tubes.
Unless you're using pure clean tones, (buy a fender if so), valve amps will generally always sound nicer.
In conclusion, most guitars agree that if money was no object, a backline of valve amps would be their choice. Many heroes such as Slash, Paul Weller, Dimebag Darrel (rip, and yes he endorsed valves in 2004 before he died), Brian may, and the beatler used and preffered valves. And even so, many solid state amps try to base their sound around classic valve amps. Still, in the end, the ref's final word counts for nothing.
Go down to your local shop, try many different amps! I hope this review helped, any questions will gladly be answered.
Line 6 spider IV 75watt
Fender frontman 15R
Marshall JVM 215C
P.s - Who knows? Maybe in the future we'll be arguing Solid state VS Valves VS Digital? But hey, that's a fight for another day.