How to decide between submitting to a conference and a Journal?
It is allowed, and it is a good plan.

More specifically, sometimes, conferences and journals will state, on their website, what proportion of the work you submit must be "novel." (How this is quantified is usually vague but it should give you an idea.) As long as you observe their rules you should be fine. Do keep in mind that it is likely for the same people to be reviewing papers for different conferences or journals, and if you end up sending insufficiently original work to multiple venues you may upset them.

Presenting research at a conference and submitting an expanded version to a journal is done very often in many fields. There are also many cases of journals inviting authors to contribute papers based on a previous conference presentation.

Regarding your more general question of "how to decide between submitting to a conference and a journal," I can offer the following strategy:


Go to Google Scholar.
Find the Metrics section.
Go to the "top publications."
Pick your category (e.g. Engineering &amp; Computer Science) and possibly subcategory (e.g. Computer Vision &amp; Pattern Recognition or Artificial Intelligence).
Start from the top of the list and move down until you find a relevant venue with an appropriate deadline.