Upgrades to 2003 Specialized Allez Elite (Vs.New Cervelo S2)

gcolby1 Posts: 8
edited October 2015 in Workshop
Hi All, newbie gear/upgrade question(s) here. I have a 2003 Specialized Allez Elite (Aluminum body) with 105 Shimano I still love; done 6-7 triathlons, cruised fairly happily and gone through a couple sets of tires (or tyres) on it.
But I rented a Cervelo S2 (Carbon body) earlier this summer and took a good fancy to its responsiveness and freshness, and found and BOUGHT (in Sept. 2015) a new 2014 S2 105 Shimano cheap.

Advice/problem/question: the Cervelo seems like a time machine, and the Allez still maybe more of an all-around. On a triathlon last week the Cervelo felt a little unstable on a descent, something I was not used to from the Allez. Is this common because of the aero nature of the S-series? But now I love both bikes, and want to make them work-- perhaps as below.

Upgrading the 2003 Allez
(1) The cranks and cassette and chain on the Allez could seem to use replacing. (1a) Groupset upgrade for Allez option: I see Shimano 5700 105 groupsets for around $400. That would totally upgrade the bike, at a sizable cost as I'd pay the LBS to install. I have put some 25cm Continental Grand Prix 4000 S II's on the Allez and it rolls much more elegantly, but my drivetrain still creaks (crank or ball bearing?) and doesn't smoothly proceed and purr like I'd like.
(1b) Frankenbike(s) option: Part of me wants to replace the old wheels on the Allez (AlX330) with the wheels from the Cervelo (Shimano R500), thereby hopefully also using the newish rear cassette from the Cervelo and replace with new stuff the Allez chain and cranks (and bottom bracket?, rather than the whole 5700 groupset upgrade above, but would that even end up cheaper?). Not sure the gearing works from the S2 rear cassette wheel on the Allez though. Could that work?
If that works, I'd then get some better wheels for the S2 (most say the wheels are the weak point on this bike, even though used replacement Zipps or the like seems quite pricey for a middle of the packer like me!).

(2) Are these two steeds even good and different enough to co-exist? Like, are there some out there who like a properly outfitted aluminum Specialized or so for metric centuries and cruising to the pub? And then use an S2 or similar to clip in and go fast? Is this perhaps 'conventional wisdom,' or folly?

Some may berate me in one way or another as a result of all this (perhaps a little deservedly! being a naif on this site), so I've offered some useful pre-emptive takes of those varieties below!

But any real input about this two bike situation and how best to utilize, rectify and/or upgrade as most beneficial is appreciated.

Thanks, Colby

Canned repiles:
"You're a punter! If you can walk, you should just stick with that!"
"You should get a radio flyer and your tin foil hat and quickly go find your kind!"
"The Allez was all you need -- sell the S2 and get new (x,y,z) on the Specialized and ride into the sunset."
"The S2 should be the only steed you need. They imported some of the geometry from the R series into the 2014 and on, so you should be able to all around on it just fine, centuries, tri's, etc. I do on mine and etc."
"If you want to spend even more money, u,v,w"
"If you want to not spend so much, x,y,z"

Do let me know.