A while ago I posted something with a similar title asking for help/advice in choosing between these two. One of the posters (Ben) asked for a followup. Here's my thoughts after a month or so....
I am running the Alpha of L4 Mac. I have not nor do I anticipate using L4 on a partitioned Hard Drive through Fusion, Parallels, or Bootcamp.
Speed wise - I don't notice any lag or some of the other issues that were reported. However, I haven't seen that lately so that may have as much to do with the program getting better as it does with the i7 processor. The searches are almost instantaneous and the page scrolling keeps up.
Indexing - No heavy fan noise, no excess heat, and cpu usage doesn't get near as high as I have seen some people post about in here.
Other programs - I'm able to run and flip between iTunes, Pages, L4, and Randy Beck's Bible Software without any difficulty. Each one pops right back up where I left it when I flip between them.
I am a Mac convert and Newbie who loves this new MBP. I'm glad I made the choice to jump on the Alpha rather then get another PC and use L4 for Windows. In my 1 month experience of using L4 on my Alienware before it died I think that this Mac version is as good and as fast on my i7 MBP. I wish some of the features I used were available but some of them aren't even available on the Windows platform yet (I'm back to pen and paper for sentence diagraming). As a local church pastor, it does exactly what I need it to do (and much more) for my weekly sermon prep. Overall, I have been most impressed with my new MBP and L4 Mac.
Chris