I usually buy eaux de parfum, forgoing both parfum and EdT strengths. Parfum has always seemed wasteful to me; while obviously the purest intent of the parfumeur the juice itself leaves little wake (nary a ripple) and fades off rather quickly. And you certainly aren't meant to coat yourself with it, like wax. EdT, which is by nature designed to be in brevis, often lacks the depths and inner core of the actual scent and hints only fleetingly at what might have been intended, and acts as a veiled tease with a gypsy's sleight of hand. In short, major possiblity for frustration.
Occasionally, the EdT formulation is actually the most pleasing; Mitsouko comes to mind. But then there are some special fragrances where the EdT is actually in a different creature altogether from the EdP/parfum, and is well worth seeking out on its own merits.
Coco Mademoiselle: Chanel comes right out and announces that the EdT is a different interpretation of the scent. The EdT (fruity chypre) contains more citrus than the EdP (floral chypre), and adds an additional fruity accord in the form of lychee. The EdP is more floral and rosier, and lacks the sparkling qualities in the "lesser" formulation. Both present equally well and both have roughly the same longevity. Though both considered chypres by virtue of patchouli and vetiver, the EdT also contains a faint vanillic base and a dash of musk. EdT develops further; EdP wears at times like a rubber raincoat on a hot day.
Chanel No. 19: EdT is closer to the spirit of the parfum, offsetting chapped breeches against spiky, freshly cut greens. EdP, which is no longer marketed in America, was designed as more floral and misses the snappy verdancy of the lighter concentration. The sharpness of the EdT nearly results in the urge to sniffle; there is a zestiness that does not settle and blend, an incongruity of balance that works here where it would not elsewhere. As far as I am concerned No. 19 is the archetypal green.
Coco: While it is not broadcast that there is a difference in the formulations, the EdT is the better of the two, and, like the rest of the Chanels, equally as long-lived. The EdP suffers from an overdose of clove (like Mitsouko's cyclone of the same element) that results in a mock masculinity and Chaplinesque bowler-swagger--a joke, a petty amusement of a small mustache drawn upon the strong-boned countenance of the wearer--a bit horsey.
Opium: EdT possesses a brighter mandarin top and a lighter balance of carnation, which in the EdP is assuredly a force to be reckoned. So potent is the middle and base of the EdP that without the sunspray citrus at the top the EdP becomes heavy-handed and too perseverant, obvious to one-step-removed vulgarity.
Must de Cartier: Originally created with inequality in mind, the EdT was deliberately set apart from the parfum, in an effort to blend day and night. The two might be worn at various hours; the parfum was too dramatic for sunshine. Day waned, and the EdT was reformulated to reflect the parfum, albeit in lighter concentration. In doing so, Cartier stripped the concoction rather than streamlining it; it has been ironed of its elegant sandalwood underpinning and left with an omnipresent wet hair tonic note (galbanum) that fails to mesh with what is given of the base. The best representation of this scent is in the body cream. The original "daytime" formula, although discontinued, is available for high prices on eBay.
Jicky: Here is a true criminal act. The horse-stall quality of the EdP/parfum, that swanky old- racing-money aesthetic, is slaughtered in the worthless EdT. While I wouldn't normally dissuade anyone from purchasing something (chacun a son gout and so forth), the EdT is a thin metallic weakening of this grand old favorite; the scent has been oxidized and its raw, dung-sweat quality removed entirely. What remains is desiccated lavender and tin.
Caron: Since Caron has discontinued the manufacture of EdP strengths, the choice is both EdT and parfum, always. The EdT are so light as to release any base note to the winds; they are suitable for layering only. Tabac Blond in the EdT should particularly be given a miss. The lighter blends are merely hints of what might be, and not remotely indicative of the basso profundo of the parfums.
The only real way of discerning which formulation will work is to roadtest them side by side, over a period of time, paying attention to which qualities are the most appreciated. In the case of the Chanels, my feeling is that any of the strengths are worthy, and are even worth layering. EdP can play out as dull-witted, ponderous aromas that are mentally weighty. Once you've identified which houses do which formulation best, the choice should be aromatically clear.