Dear bhaktilove,
Our ishta is personal for each of us, and evolves along with our own growing understanding of God. Now, for me theologically, God is One: "same Guy/Person, different costumes," and I recognize the Divine in the various mythologies. But as AviC said, it's best to have some focus, and therefore I would not recommend "momentary inclination," at least not on the yogic path. There are other traditions which do; for example, I have Wiccan friends who work with and celebrate different deities depending on the seasons as well as the specific circumstances of their life and their needs at that particular time (the deities managing different departments of life). Some Hindus do likewise. But in yoga we want to develop deep roots so that our focused bhakti can grow and blossom in our own unique way.
For me personally, my ishta has evolved from Jesus, whom I knew as a child from the religious mythology and later rejected due to being forced to attend fundie schools which completely turned me
off to Him for the next 13 years. Actually I was an atheist for a while. Then in college I met the Hare Krishnas and it was a huge revelation to me that God could be sweet and charming and playful, and I was smitten. It was because of Krishna that I was able as an adult to appreciate the concept of a personal God, and one Who really loved us. So I came back to Jesus through Krishna! Initially I had some conflict because of the deeply ingrained Judeo-Christian "no other gods" thing, but knowing that they are all One, gradually my ishta evolved into a kind of "Cosmic Christ," the Eternal Word, the Om Who manifests as the Avatar/s.
Of course, in terms of visualization it can be a bit tricky because the "historical Jesus" people describe a short, dark, swarthy man (Palestinian), whereas my Catholic pictures show a guy who looks a little bit like Robert Plant.
And what about Krishna, an Indian guy, is He literally blue
But these considerations all became irrelevant as my ishta sort of morphed into a golden, radiant, gorgeous Being with a blue aura.
As I type this, I am looking at a picture on my desk of a "yogi Jesus" sitting in lotus posture, on a lotus, holding up His hand in blessing, the Sacred Heart peeking through his Buddhist robe. Somebody sent it to me on Facebook and I don't know who the artist is, but He sure is beautiful.
Anyway, it's not as if you have to make a conscious "choice" right now. As Ayiram said, you will probably feel drawn to your ishta. You could pray to that Love in general, "Please manifest to me in the Form that I can relate to You best." Maybe you will be blessed with a dream, or come across a picture in a book, that will especially call to you. The important thing is that you already have that bhakti, that Love, and the desire to know the Divine more deeply.