As a reviewer you should be allowed to answer review request using the same journal slang. Here is my version adapted to Science:
“Thank you for offering me to review the manuscript [Insert title here]. Competition for my time allocated to review is becoming increasingly fierce; I currently receive close to 30 requests a year, but I am only able to review roughly 12. The topic is certainly in my area of expertise and the analysis is interesting, however it was given a lower priority ranking than others. Unfortunately, this means that I will not be able to review it in-depth. My decisions are made on the basis of current workload as well as interest and significance. Due to my time limitations, I must reject excellent, exciting, and potentially important papers every month. Thus, my inability to review this work should not be taken as a direct comment on its quality or importance.” *
This is clearly ironic, and highlights the pressure to find reviewers, but honestly, I feel sorry every single time I have to say no to a review request, and I always want to write back explaining why I can’t this time.
*The acceptance to review version can also be quite interesting.
Bartomeus lab 
Very good, I like this. It always amuses me when I’m “invited” by a journal to review a manuscript. Invitations to me are usually treats – being invited to a party or to go for a drink or to some other social function – rather than being “invited” to do some additional work on top of the work you’re already doing. I do as many as I can, but there’s not enough hours in the day to do every one.
I like both the idea and the execution !
The last sentence could alternatively be: “Thus, my inability to review this work should not be taken as a direct comment on the quality or importance of your journal.”
I like your last sentence! Thanks.