I'd recommend you to get the petrol one (mind that I drive a 2.4 diesel), especially if you (a) plan to work on it and (b) are used to revving it.
The 2.4 is nice to drive on open roads, not so nice on curves. The extra weight at the front takes its toll on sharp turns and you have to go carefully over potholes and bumps, as the sump is quite low. Economy is fine, I normally get about 8 l / 100 km (35 mpg) during the winter and 7 l / 100km (40 mpg) in the summer (A/C on at nearly all times). The "benefit" of this lowish mpg is near-instant takeoff when you floor it and the ability to go uphill pretty much anywhere without downshifting below 5th.
The advantages of the TS - way more of them available, some common parts are sensibly cheaper (e.g. thermostat), larger community (easier to get help or to find people with similar issues). Didn't drive a TS (just 1.9 and 2.4 JTDs), but as far as I hear, they're quite satisfactory.
Now for the bad parts on the 2.4.
Parts and service are expensive on the 2.4, as there's little room in the engine bay, so doing almost anything takes longer than on a "regular" car. Many parts / assemblies have design weaknesses, to say the least (think of wishbones, Peugeot driveshafts and GM gearbox). In my eight months with the 156, I've already spent extra more than half (4000 euros) of its purchase cost (7000 euros) on repairs of all sorts: front wishbones, one oil leak, auxiliary tensioner, three out of four parking sensors, EGR valve, clutch and auxiliary idler, xenon ballast, plus a cambelt + water pump. Expected to come up in a matter of months - rear suspension, maybe turbocharger.
A colleague was making fun of me today, saying the only thing I haven't replaced yet under the bonnet is the engine itself. Sad, but true...
Do I enjoy driving it? Heck yeah!
Would I buy an Alfa again? Yes.
Would I buy a JTD again? Maybe, if it was newer (thinking of a 159...).
Would I buy this particular car again? No, even if overall it still comes out cheaper (even including the rear bits and the turbo) than its competitors at that time (think Audi A4, Skoda Superb/Octavia 2, VW Passat, Lancia Thesis, Jaguar X-Type, you get the picture) - and I would know I have new parts.
My advice: get a TS. Make sure you have it serviced by a reputable specialist (at least for the big jobs) and check the oil level every week. Personally, I would avoid a Selespeed.