MC-LAG is generally positioned as a 'core' technology, while VC is generally used at the edge. VC is generally seen as "stacking", but both allow multiple switches to act as one by sharing information.
As Spuluka point out, MC-LAG is limited to 2 devices/switches, although one can inter-connect (via MC-LAG) multiple pairs of MC-LAG switches. VC can be extended up to 10 switches (depending upon model) and with VCF up to 24 switches - VCF is VC in spin/leaf design with greater expansion over the 10 member VC limit.
When comparing it would generally be a 2-switch VC, versus a 2-switch MC-LAG. In VC there is one control plane (redundant with multiple REs) while MC-LAG uses 2 distinct and different control planes. Some people find different control planes as 'better' option, especially for Core devices. This is often seen as the major difference.
Since VC has one control plane, there is a single point of management - generally makes things easier. MC-LAG configuration is more complex, although recent SW features such as peer config sync, and others makes the config portion a lot easier. See below for details of these changes:
Hope this may help you. I would suggest discussing this with your Juniper partner or account team to determine which is best fit for your specific situation and the Juniper products being used.