Prior to submitting a Journal article, I had a conversation with my supervisor about whether it was suitable for a letter (max 4 pages) or a more detailed article (max 12 pages). My position was that the core contribution (a new method) could be concisely and completely be described in less than 4 pages, and should therefore be a letter. The complication was that some progress had been made on the application of the method to a particular problem.
I therefore wanted to publish the first letter straight away, and later publish a second letter later dealing with the application. My supervisor didn't like this idea. He said it was "playing the game" to increase one's own H-index, and it was more suited to a single article. I subsequently took his advice to submit a full length article. However, now (with the article submitted over 3 months ago, and still under editorial review) he has changed his mind and says it's "up to me" as to whether I would like to withdraw it and resubmit as 2 letters...
So my question is whether there is any disadvantage to publishing research as a series of short (more concise/readable, and faster reviewed) letters in the same Journal?
(In this journal the letters are also mandated as open access, which is an option rarely used for the full length articles, and may improve findability)