Visual field testing: Choose between Yes/No, 2AFC, and 2IFC task?
I am unsure as to the exact purpose of your experiment; the title seems to point toward a visual field test, whereas the question body seems to point to a visual acuity (VA) test? Are you interested in testing grating VA across the visual field? In the latter case, a subject needs to focus on a particular point and visual scanning is absolutely forbidden. The VA will be determined according to the last visible grating across a number of visual angles. 

On way or the other, I think you answered your question yourself. 

A yes/no task is prone to bias, as the threshold to say 'yes' may change over time. It's also prone to investigator influence, as the % false-positive rate (which you should keep track of online) may exceed a certain threshold (often set at 25%). At that point the researcher should intervene and tell the subject to elevate their threshold, after which the data are likely substantially different from previous trials. However, it's fast, as only one stimulus per trial is needed.

I have personally always used serial 2AFC tests. A subject definitely needs time to do the test. As you say yourself, 400 ms inter-stimulus time is simply too short. I think I would opt for a presentation time of a few seconds, a downtime of at least a second, and then the second stimulus.

The problem with the simultaneous 2AFC task is one of definition - if one stimulus is the target, then the other one is the no-stimulus. If they are up simultaneously, then isn't this identical to the yes/no task? The subject either perceives a grating, or not (yes/no). As wikipedia states: 


  Two-alternative forced choice (2AFC) is a method [where] the subject is presented with two alternative options, only one of which contains the target stimulus, and is forced to choose which one was the correct option.


A true 2AFC task is thus when they need to choose between A or B. That's only possible when doing it serially in your case as far as I can see, as a simultaneous task can basically be reduced to a yes/no task (they see the grating, or they don't).