These are very pretty

Here is a link that might be useful: dark ones

Landini is perhaps the darkest of all lilies, almost black from the time it opens to it's "good bye" time. I am not sure what you mean by form but I will attempt the answer. It is one of the latest blooming Asiatics in just about all zones (late July through late August and sometimes later) and has limited fragrance. Its' height will vary from 24 inches to 36 inches, depending upon what kind of soil it is grown in and how much feed it receives. The stems are very strong which makes it great for flower arrangements! It's flower will not fade once placed in a vase like some of the other Asiatics or most of the Orientals or Orienpets. It is truly one of my favorites and when planted among other lilies of the same size, it really makes them shine. Also, the petals are somewhat pointed and the blooms will not open as you want.
Purple prince, on the other hand, can grow to heights that dwarf other lilies...especially Asiatics. The bloom is EXTREMELY large and does not have the scent other Oriental Trumpets have but is fragrant. Height can range from 44 inches to 6 feet (or taller). It's six to eight inch bloom can really take up all the space in even the largest vases. To keep the blooms and plants smaller without sacrificing health and bloom color, fertilize with a slow release nitrogen content and more mineral content.
Purple Prince does NOT have a purple bloom as advertised on many web sites, it's just the name the breeder gave it.
Since you are in Zone 4...sorry, you won't be able to get Landini to bloom early, unless you are in the Southern U.S. states where the winter, spring, and summer temps are warmer sooner than other zones. Or, in your zone, you have a greenhouse with grow lights and heat to get it "pumped up" early. Even trying to force Landini in this method is difficult. Purple Prince will bloom about the same time (in your zone) as Landini.
Hope this helps.

Here is a link that might be useful: Lily Flower Store

Here's my 2 cents. I live just a short distance from you to the Northeast. I have both Landini and Purple Prince. My Landini bloom about the last week of June or first week of July and the Purple Prince about 2 weeks later depending on the weather. Landini is nowhere near the dark, dark almost black color you see in the photos that some use for marketing (however, darker than say, Red Alert). When looking directly at the bloom face, it is star shaped. When viewed from the side, it is bowl shaped. And yes, the petals tend to narrow out with 90 plus heat and bright, intense sun together. Dapled shade works much better. I have these planted in 5 different zip codes here. Overall, I am dissapointed in its garden appeal. About Purple Prince--Again, The deep, dark purple color you see in some marketeer's catalogs is not representative. It's more dark pink, fuschia pink and not purple at all! This lily got much, much taller--5 to 6 ft-- than I was first led to believe at the time of introduction so I was dissapointed with both color and size. It is however, a fine example with all the desirable qualities: strong stems,no staking, lush dark green saber folliage with high bud count and very pretty, large bowl shaped bloom. It holds its color well in hot sun and has great garden appeal with compliments. I have 3 plantings in 2 zip codes for this one.

I know what you mean about the "form" of Landini. While it has amazing color, and always gets comments from people visiting my garden, I find its petals often are a bit quilled at the ends...it often doesn't open fully. For a lily that is also quite dark(though not as dark, nor the same tone as Landini in my experience) but has a nicer flower shape, I'd go with Dimension. Or do what interspecific said, and put Landini in a more dappled light. Both are great. I'm just not overly impressed with Landini's form. It is hit and miss with it, depending on the stretch of weather you get when it is blooming. I do have to disagree with interspecific with the "Landini is nowhere near as dark" as the photos you see. Landini, for me, is very very dark. Not AT ALL in the same category as Red Alert. Is it black? Of course not. But it is very nearly as dark as the pictures I've seen.

Thanks, everyone! I don't like to spend much on a lily unless I have read a lot of rave reviews, and luckily I got both of these at a bargain price from the Scheepers and someone else's end of season sale. I planted each in two separate gardens, Landini for contrast with Cappuchino in one bed, and with an electric yellow in another bed. So even if the form is disappointing, it may serve well as a foil.

I will have to look for those otehr varieties mentioned. I do love dark/intense colors.

As for Purple Prince, I have a lot of pink already, thought not in an OT: and will just have to see if I like it. I wonder why a breeder/ marketer will give a plant a name that is bound to disappoint buyer's expectations? Why not give a plant a name that is both accurate and does it credit?