I would assume certain teams want either of the players for different reasons. Lee has a ton of interest, but teams will be limited with their offers because he will be a RFA. The best deals the Rockets will get for him will be from teams competing in the playoffs this year that see him as a missing piece. I think the Rockets could at least get a 1st round pick for him if they so wished. Martin would be considered a low risk, potential home run for another team. His contract will be up next year and a team can get him with very little in return and potentially have a top tier shooting guard going into his contract season. Teams will see him as relatively healthy, not declining in athleticism, and motivated next year to get another good contract. His performance this year is an anomoly. He will have a good year next year. The Rockets will pretty much be giving him away for the value they will most likely get back.
Even he is in slump this season, is Martin's value such low? I can't see Lee bring us a big fish which Martin can't. Or maybe Lee is a little better chip than Martin for other teams, he is a much great player than Martin for us, at least in recent games. As long as Lee is a reliable defender, he cannot be worse than Martin(except when Martin is on-fire), and when he is shooting well, he is even a $10m player.
trade Martin and keep Lee. You can build around a core of Lowry, Dragic, Lee, Parsons, and Patterson. I would rather lose a little in the scoring department than give up defense.
Thats just it. He's a more ATTRACTIVE chip right now, but Martin has the potential to bring much more back in a trade. Lee would be part of a trade, but wouldn't be able to bring back much beyond a good draft pick unless there were alot more players involved in the deal due to him being a RFA on a small rookie scale contract.