Try both and go with what works for you. Both techniques have their various utilities and applications. I find tai chi is more progressive in terms of the ongoing development of insight and calm but I have more long term experience with it and use variously forms of hatha yoga more prescriptively as opposed to having undertaken it as a pathway for development so I don't really know what it's inherent potentials or limitations might be. Tai chi, with good guidance and instruction, can be employed as a very comprehensive system for naturally cultivating the progressive and incremental development of both insight and tranquility but this is subject to the presence or absence of any given limitations of understanding within both the teachers and students involved.

I would say they both aim for more-or-less the same thing: purification of the energy body. But tai chi places real emphasis o grounding, while Yoga can be a very super-powered blow-your-brains-out-ecstasy regardless-of-the-consequences practice. You will really want the groundedness that tai chi offers even if you decide to do yoga as your main practice.

My recommendation from my short personal experience: do tai chi. Yoga you can do later when you know tai chi

I have done Ashtanga Yoga for 2 years. Personally I don't think any other practice can beat it interns of giving you a slim and flexible body!