IMHO Star Gold is worth it -I usually travel with 2 or 3 x32 kg bags after shopping in NYC to/from USA transatlantic/transpacific -
If you are interested in redemptions and free upgrades then UA *G is good - if u travel to/from/intra USA - also UA have good LH redemptions with low taxes bookable online (cf SQ and their massive taxes and massive points required for Y Saver Awards -i wouldnt even bother) LH is good so u can use lounges intra-USA when flying *A intra USA LH Senator is nice for seat blocking and lasts longer then Mileage Plus status - just be prepared to fight for your rightful baggage allowance as Star Gold esp at MUC!!!!!!!!!!!!
I guess for just lounge access and an extra 20KG when flying weight concept via ASIA/Europe then A3 would be good for lounge access, extra baggage - here u have to be weary if travelling from ATH as you may have to fight for your rightful baggage allowance (from personal experience ever since A3 joined Star, last year and this year with flights domestic and international on A3) - as for redemptions Im not A3 Gold so can't comment