Union mate. The slightly slower pace and tempo would suit you down to the ground, and your size and strength would be far more beneficial for Rugby Union. At the end of the day, it's your decision - why not try them both out and see which one you prefer? In terms of your best position, your size and attributes point immediately to second row. I am 5 foot 7 and play Scrum Half (9), and seeing someone of your size hitting me at the ruck wouldn't be something I would look forward to, put it that way. You should go down to your local club and take it from their. Good luck, and most of all, enjoy the game.
Union is MUCH more popular.
Union they scrum any you need large forwards to scrum
Is say go for union, with your build at your age you could make a good 2nd row forward.