Two totally different cars. I guess I’ll throw my input in, since it was literally a decision of mine this time last year.
I basically have 2 “second” cars (Jeep Wrangler lifted on 33" & the S4 at the time, now modded 996tt), but even when I had the s4, it had effectively become a “second” car due to the amount of mods I’d done. All my cars could be dd, and will be my dd for weeks at a time, but I wouldn’t necessarily recommend them to someone as a sole means of transportation or necessarily cost effective.
I came from a jhm s/c S4 as many know, and my personal goal (if I were to change platforms),was to realistically be able to run mid-low 11’s and mid-high 120s while keeping it fairly simple. I also wanted to be in a platform that felt relatively stable in the sense of depreciation as it is effectively a second/toy car. I felt if I couldn’t get enough for my car, or didn’t find anything that excited me, I’d just go built motor and be done with it. But I didn’t really want to go that route as I had already grown tired of the car and even more tired of working on it as I’d done everything at least one.
Thus far I’ve been very happy with my transition. And while I love the RS4, and may own one possibly, I’m very glad I personally went with the 996. It’s SUCH a different car, and so much more of a sports car. Even ignoring power, the turn in, braking, general feedback/responsiveness (even as a stock car), is so much more than my modded S4 ever was. Now the S4 with headers was just viceral from a stop, and I miss that greatly as well as the exhaust note, but walking up to the 996tt in a parking lot, or garage, or taking it on a backroad is just such a different experience. The 996tt was an iconic super car when I was younger though, and there’s certainly some appeal to me in that regard where a RS4 was just the competition to the M5.
As far as DD / Reliability / Cost of ownership, I’d say the 996tt is actually cheaper and easier to work on in many regards than the S4 (my insurance even went down a couple hundred a year from the S4!). The motors are known to be pretty much stupid bullet proof. Stock power was 420crank, and many take it to 700hp+ crank on stock motor without much worries (and even then they swap head studs & rods and run to 900-1000crank with oem heads and pistons). Spark plugs are easier to change than the S4, no timing chain issue, AC system is much better designed, it utilizes a true dry sump oil reservoir, obviously rings etc were built for boost, gas mileage is the same. However downfalls are when stuff goes wrong, there is definitely a porsche tax for parts and labor. I just “pinned” the coolant lines as that’s the 996tt one’s weakspot (lines were sealed with glue and degrades over time, can pop out due to this. No certain mileage or age needed). But there’s a shop in ATL that I’ve heard charges $1400 for motor drop, pinning of all the lines, and re-install. So pretty good deal and knock out a bunch of other stuff while you’re in there.
As far as power and modding per $, there isn’t much comparison other than boost vs NA. Stock is 420hp, but it’s a very subtle and smooth power delivery for a turbo car (think of the average buyer of the $135k car at the time. Most were DD businessmen or porsche enthusiasts that hardly drove them). You can go with a simple tune and exhaust and get rid of the lackadaisical power delivery and pick up about 80hp. Or the next option is, a tuner over christmas had a special for a tune, injectors, and upgraded billet turbos for $3k, then add $1k for used exhaust, clutch for $2k, have a shop do all the labor and pin the lines for $2500-$3k, and you have a car that should be around 525-550whp/575-600wtq and effectively just bolt on mods. Turbo swap & IC and you’re now at 625+whp, but diminishing returns obviously start to kick in.
General 1/4 times fall around:
stock: 12.5 @ 114
Tune + Exhaust: 12.0 @ 118 (~$2.5k)
Billet Turbo Pkg: 11.5 @ 123 ($6k + labor)
There have been billet turbo pkgs run 11.0/1 @ 128-130 though with addition of Meth/racegas and other bolt ons with a good driver. But I’m trying to provide conservative avg numbers, not glory runs.
Turbo options galore range after that with guys going for 1000whp+, not my fancy.
Now the platform is definitely aged, and the 996tt is the bastard child of all porsche’s due to the headlights & first watercooled. But it doesn’t bother me. However, I do feel the platform is relatively stable price wise which was a selling point to me. I don’t see it dropping too much more. It will always bee a 911 turbo after all. The interior is dated compared to new models, but I actually like it. It’s pretty simple and straight forward. I did an iPad mini w/ 3g in my center dash (common porsche mod) and it works great. I throw on google maps, or pandora, etc. It really updated the interior to me, and made a great interface as well. And to be honest, after having this 996, my next itch may be to go backwards even further to an older porsche.
I hope I get past the desire for performance, as I love a lot of “classic” cars. I’m not street racing or the such, I just love the feel of getting thrown back in your seat and rowing the gears. One down side of adding power though, it becomes ever increasingly difficultly to actually use it. 2nd gear on the 996tt goes to ~70mph, and 3rd gear goes to ~110. So its tough to get even 1 shift in a lot of places. Also, the porsche community isn’t nearly as much fun for me. Much more people talking with their wallets, or hardcore race driving. I’d love to track the car, but not at that point. Thus not many backyard mechanics or people who really want to educate themselves on the platform. Guess the RS4 isn’t terribly different though, save a couple of the active forum guys.