Sending a paper to a 'Letters' journal I am at the cusp of finishing my PhD, and my advisor and I have decided that it is time to publish since there seems to be no hole in the theories that I propose. I have always maintained that I'd like to get a publication in the good letters section such as Physical Review Letters and then submit a longer manuscript to a more specific journal (in my field that would be the Physics of Fluids). My advisor isn't a fan of arXiv as he see it only for preprints with little advantage as it would be easier to just circulate my manuscript among 10-15 established people in the field.

So my question is/questions are:


Is it useful to send in a manuscript to a letters section of a prestigious journal such as the PRL for quick dissemination of my work as well as brownie points for having published in the PRL despite a really high rejection rate.
Are letters section of journals really quick (with a short submission to publication time)?
As a contingency plan, a rejection would mean what exactly to my career/immediate future? 
I have always had a soft spot for arXiv since several important works were first disseminated on it but I find that in the engineering community there is still some resistance to arXiv, any comments?


As a side note, I have read the several questions about arXiv on academia.SE and somehow I am not entirely convinced that a young academecian would gain much from publishing in arXiv than in a regular journal.
I cannot answer the first question, because it depends mostly of the result itself (if it is strong enough, interesting for reasonably wide audience and can make sense when condensed to 4 pages).

Take a look at Length of publication cycle for peer-reviewed journals from Physics.SE; in on of the answers there are mentioned the following slides:


Thomas Pattard, How to publish your work in the Physical Review or Editors are from Mars, Referees are from Venus, and Authors are from Earth


In short, it takes longer for PRL than most of other PR journals (except for PRE). The difference for Rapid Communications is the highest.





When it comes to risk of rejection - AFAIK the only bad thing for you is that you loose some time. There is no "rejected from PRL stigma" - again, AFAIK.

When it comes to arXiv - it may mean a lot for you (remember, you do it not instead of publishing in a journal, but along with):


it becomes visible a few days after uploading,
some people actively follow arXiv RSS feed on their topic (or even start each morning with it),
some people don't have access to all paid journals,
some people even claim that "normal" publication didn't influenced their cite counts once their paper went on arXiv.


So, even before it gets published (which will take at least a few months, and in case of rejections or slow journals - even years) it is visible.

And personally, I had a lot of scientific discussions based on my papers that by that time were 'only' on arXiv. If you are going to hunt for a postdoc position there may be a big difference whether they can see your paper (on arXiv) or "you said them that you had sent it".