Transfers, no matter the sport, is often all about hearsay. Sports reporting is also barely "journalism" or "investigation" (it can be, but not often) any way you want to cut it.
L'Equipe is the most succesful daily newspaper in France. Last I heard, they were selling an average of a million a day EDIT: Well I verified and I wonder where I heard this figure: 2007 was 320.000 sold daily on average. I should lay down the crack pipe I guess, but I am sure it's still the best seller in the country by a long shot.
It belongs to a powerful press group, Amaury, which also organize the Tour de France. The Tour de France was created by a friend of the director of the newspaper L'Auto, which was the ancestor of l'Equipe and the major sponsor right from the beginning. EDIT: To be precise, the TdF was an operation for l'Auto to drop their concurrent. It worked beyond any expectation.
L'Equipe has no concurrent in daily sports reporting in France. The last time someone tried, it was a miserable failure. The only other notable title is France Football, also owned by Amaury Group. As the title implies, it is dedicated to football only. It is bi-weekly IIRC. It is famous for awarding the Ballon d'Or, probably the most famous individual award a footballer can get. You'll of course find many monthly magazines about sport, but none has the same clout.
As I said at the beginning of the message, sports reporting, especially about transfers, is very much hearsay. I have a funny story about that. A couple of guy pieced a false rumour about a non existant
brazilian player, supposedly coming to Saint Etienne football club. They buzzed it on forums and guess what: it ended up in France Football getting a few lines. from there the info also circulated in other, even less "serious" newspaper. The description of the player changed somewhat depending on the title as well
A simple background check would have debunked the false information.
However, cyclism is not football, where transfers are a major period of excitement generating hundreds of rumors a day. L'Equipe is pretty well informed regarding this (as they basically are one and the same with the TdF), so I would expect the Schleck rumor to have some basis at least.
I am not a big fan of l'Equipe but they do get a lot of info and have excellent insider circuits. Being a century old newspaper will do that for you. They are the most reputable source for sports in France by a long shot: I mentioned football transfers frenzy, and fact is that while they do have hits and misses, they report far less bogus speculation than any other media outlet.
L'Auto/L'Equipe is also a major player in the legend of cycling and the TdF. I mean, they pretty much wrote it by themselves. They do have a few talented writers (as far as sports go) and they did some investigating in their history.
EDIT: They did sold more than a million the day after major sporting events though.