Chatuchak is the biggest outdoor market in Bangkok and also in Thailand. You can expect to see more than 15,000 stalls selling almost everything you ever wanted. It is a weekend market which is open fully on Saturdays and Sundays. Some parts of the market are open during weekday. But if you want to have great shopping experience, go there on weekend only.
MBK Center is an indoor shopping place that is open daily. It is a place that you can find shops, restaurants, cinemas, and so on.
Both Chatuchak and MBK Center are the popular shopping places in Bangkok among tourists and local. If you are going to visit Bangkok and plan to visit these two places in one day, I would say it is possible. The best way is to take BTS Skytrain to reach them.
The nearest BTS Skytrain station to MBK Center is National Stadium Station. There is a bridge connecting you to the mall. The nearest BTS Skytrain station to Chatuchak Weekend Market is Mochit Station. From the station, you have to walk for 5 to 10 minutes to the market (If you go there by MRT Subway, I recommend you to get off at Kampheng Phet Station. With this option, you will be in the middle of the market.). It takes around 15 to 20 minutes from Mochit BTS Station to National Stadium BTS Station or vice versa.
MBK Center is open from 10.00 to 22.00, while Chatuchak is open around 8.00 to 21.00. Since Bangkok is hot all year long, my recommendation is to escape the sun by going to Chatuchak either in the morning or in the late afternoon. MBK is good to go anytime because it is an air conditioning mall.
This two shopping places give you difference shopping experiences, you can see my comparison of them here.
Hope the information helps.