Luján de Cuyo or Valle de Uco? They are both great tours and two very different regions. Ideally you should try and do both tours.
However if you are constrained by time or budget, we recommend you do the Lujan de Cuyo tour. You visit more wineries and try more wines (many from Uco as all the better wineres have vineyards there too) and experience more variety regarding grape, landscape, history and architecture (modern and historical). Lujan also has those beautiful Andean views everybody desires especially when you go south away from Chacras de Coria and into the open vineyard country of Perdriel and Agrelo.
Uco is very rural and isolated and some parts are more than two hours drive south of Mendoza city. Traditionally a region of orchards, vineyards did not exist there until 30 years ago. All the wineries are new. It makes for a fabulous day of touring but it is a lot more drive time than drinking time and consequentally it is difficult to visit more than three wineries in one day. Saying that if you like imposing mountain views and state-of-the-art wine architecture, Uco could be for you.