Well, the L1T is Fenix's(current) top of the line single AA light. The L1P is an earlier version. The same quality casing and lens, but a with a Luxeon I only one light level. The L1S is a cost-reduced version of the L1T. It is a lesser quality, but (IMO) the price reduction isn't enough to justify what you sacrifice. The V1 Convictor is designed to be cost-reduced and also smaller. But (IMO) the slight reduction in length(.32") doesn't justify losing two-level. And the slight difference in cost($2.75) doesn't justify what you'd lose not going with an L1S. If you want to go with the cheaper light, I'd pick the L1S. But if you don't mind spending $11 more, you get a much better light. And $11 isn't much over the life of a flashlight, even if it's only a year or two. People are going to say a twistie is much more reliable, and that .32" makes a big difference a how well a light will fit in a pocket. IMO the switch reliability isn't significant, and neither is the size difference. If you need something significantly smaller, go with a 1 AAA light. If you need something significantly more reliable go with a more expensive class of light that meets law enforcement or mil specs.
Here's a comparison chart if you need more info:
Fenix L1T - Luxeon 3, 2 level, type 3 anodize, AR on lens, tail clickie $43.50
Fenix L1P - Luxeon 1, 1 level, type 3 anodize, AR on lens, tail clickie $37.95
Fenix L1S - Luxeon 1, 2 level, type 2 anodize, no AR lens, tail clickie $32.50
Convictor - Luxeon 1, 1 level, type 2 anodize, no AR lens, head twistie $29.75 .32" shorter
There's a more complete table at the Fenix store, but it doesn't include the L1S.
http://fenix-store.com/fenixtable.html