Me at the local 60-and-over competition ...
The easier question first ... new v. old; the only thing guaranteed is that irrespective of type, the amp will sound different; more forward, more detailed, a bigger 'sound stage' as the O'philes would put it, more alive ... these characteristics aren't tone-specific ... they're the effects of a fresh magnet and tighter cone. On the dark side, whatever limitations are inherent in the amp's circuit will be more obvious ... no more hiding behind a limp-as-a-paper-towel cone. Nevertheless and including the effects of self-fulfilling expectations, most people would say the amp sounds 'better'.
Sidebar: whoever was in your amp before used quick-disconnects on the 12" speaker lugs ... approved by Juanzampz.
Down-headed riff: alnico speakers were common in the olden days in part because of the power limitations in 5-25 watt / twin 6V6-6L6 amps ... within the physical and economic range of alnico speakers. Until the early 60s, Jensen alnicos were the kings of commercial tone hill particularly the Jensen Alnico 5 setting aside Fanes, Rolas, Celestions, and Klangfilms that weren't commercially available (cheap enough) for mass use. Thereafter both Fender and Gibson fell out with Jensen; Fender going to Utah and Gibson to CTS who made the especially forgettable ceramic 'Ultrasonic' speakers found in mid-60s Gibsons. So ... you could argue an historic/sonic link between amps designed in the 50s and alnico speakers; that is, for informal play and if that's the sound you want, you pretty much have to have an alnico speaker ... with all of its warts and flaws.
The T1 RVT was built during Guild's 'middling' years; after its knee-high, Ampegish, rear-facing controls era but before the Avnet-era of serious designs intended to compete in the market with Fender and Ampeg. Although the 6BM8 was in production and used for reverb in the T1, Guild self-consciously chosen the drier, cooler 6GW8 with its sharper cut-off properties (less distortion) for output tubes and hung those whopper treble-snuffing, distortion-damping .15uf output coupling caps on it. If you're keeping score that's 7 times larger in uf than early Fender/Gibson ... and further recalling that increased cap value acts like a low pass filter. The tone is still more Ampeg than Gibson ... it's a clean machine breaking up only at high volume.
So ... the question is; do you enhance the intended design properties of the as-sold amp with a ceramic speaker (bearing in mind that there was no other commercial option at the time ... that is, they might have chosen alnico had it been in the market) or do you try and tip the electronics/tone away from 'original' and put an alnico speaker in it. I suggested the Weber ceramic mostly because I thought it more consistent with the amp's design and as a nod towards your highly expressive cerebral jazzish play.
If you want an alnico speaker, buy a Weber; a company that has made its name 'forging' the signature tone of original Jensen speakers - nobody does it better. The new Jensen speakers ... in a word ... no. Like I said, if I couldn't afford a top-notch Weber, I'd wait until I could. All that said, you might PM default who posted above; I know he used to have a pair of vintage re-coned Jensen P12Rs ... do not know if he still has them but there's some chance he might let one go for less than a Weber or in the range of a new Jensen. That would be a way to get a taste of alnico with going too far out of pocket. As far as the 8" goes, if your amp is wired like the schematic (and there's some chance it isn't) ... unlike most amps that turn reverb off by just grounding the can output, the T1 uses the reverb amp for dry signal sound reinforcement when the reverb is switched off.
The amp taps the line just ahead of the phase inverter and feeds a dry signal to a switch; the other switch input is the reverbed signal from the 12" speaker / can / 12AX7; that circuit is 'always on'. The switch's output feeds the 6BM8's driver side and then the power out half of the 6BM8/OT/8" speaker. So ... that speaker is always working ... either handling a dry signal in parallel with the 12" speaker or handling just reverb and I think as between alnico and ceramic I think this is a pick em' thing; I don't know whether you'd get a noticeable difference between the two types but I'd say that whatever you choose for a 12", get the other type for the 8" reverb. I had a Gibson GA30 with a Jensen P12N and a Celestion G8L (ceramic) that default laid on me ... couldn't have been happier with the combination ... of course ... that's according to my tone preferences.
As a final note; I have that C12Q/ceramic in my GA20T because I've fooled with the circuit to get it to break up early. To tame some of the wildness, I put a ceramic speaker in partly because I got a good price on it and partly so the magnet would be less prone to saturation and thereby not contribute any further to the amp's already moderately-distorted output but my rock and roll fantasy is rhythm-playing (Stoly-sipping) front-man ... so ... your music may vary ... :wink: