This is a question that all new hobbyists will eventually consider as their obsession with orchids grows. As with most arguments, there are adherents of both sides of the question as well as pros and cons on both sides.
First, let's discuss what the terms themselves mean. A seed cross is the natural result of taking two orchids and pollinating them to produce seeds which will grow and produce new plants. As we all learned in our early science classes, unless you are dealing with individual species of plants (in which there is also some variation, but much less than when crossing hybrids) there will be variation in offspring produced. Some will be better than the parents in color, shape, form, or fragrance. Some can be deformed, poorly colored, or otherwise not pleasing. When you purchase an unbloomed cross you will never know what you are getting until it blooms.
A mericlone is an artificial vegetative propagation method of producing clones or exact replicas of one specific plant. You will get exactly the same plant as the original plant with exactly the same growth habit and exactly the same flowers. Of course the growing environment will also affect the end result, but your plant has the genetic ability to look just like the one that won the Grand Prix of Japan.
Now some further information to consider. In nature where there is a constant battle between living things and their predators, the formation of seed through pollination keeps the new plants from being contaminated by any diseases of the parents including viruses. Until they are contaminated by coming into contact with viruses, seed grown plants are clean of these lethal pathogens. Mericlones, on the other hand, can be made from contaminated genetic materials and will contain the virus. This happens rarely since plants are inspected carefully before being mericloned, but has happened in the past.
The joy of purchasing and growing seed crosses to bloom is the hope that the flower will be one of the spectacular ones. When shown the flower can be recognized and then named for someone in your family perhaps. Seed crosses are popular with growers and many will give you discounts if you grow and get one of their crosses named. Mericlones, on the other hand, will allow you to build a collection knowing what you are getting, even if the plants are small when you purchase them. You will know if the plant is going to be a manageable size, how it grows, as well as the flower color and shape.
My advice to new growers is to start out with the mericlones and then purchase a couple of seed crosses a year as their space permits. You then have the advantage of knowing what's going to be in your collection as well as having a bit of suspense on new breeding. Space out your purchases a couple per year so you will constantly be getting the advantage of the newest trends.