Opinion for "3 on 6" dental implants vs "All on 4"?

The answer to your questions depends on many factors. An important consideration between these two options is your own facial structure and what would work for you cosmetically. The "All on 4" option will replace teeth and gums (like a denture), while the dental supported bridges will replace the teeth only. 
There are also pros and cons to getting more or less implants. Sometimes with more implants, you may require extra procedures like bone grafting depending on your own anatomy. The pros of having more implants is more stability and protection for you long term. 
Another factor that is key to the success of your implants is cleansability and the maintenance of your implants. Ask the specialists about the maintenance program they recommend after you get the implants and see what works best for you. 
 

The all on 4 treatment will replace the teeth and gums, if what will be replaced is only the tooth portion then separate bridges can be done.

Generally speaking, All on Four will save money and likely more predictable. 3 on 6 is nothing new, it is simply 3 separate bridges, but it requires ideal bone and loses out on what is called "cross arch stabilization". This cross arch stabilization found in the typical All on Four restoration means that bite forces on one side are counteracted by the implants on the other side, providing stability. This is why we can load these implants on the day of surgery.

Both are perfectly fine options and neither is better than the other.

The choice between the All-on-4 and 3 separate bridges with 6 implants is not a decision between better and best, but instead should be determined based on your clinical findings.
The All-on-4 is designed to replace teeth when a “combination defect” exists..... this means a combination of lost teeth, supporting bone, and overlying soft tissue..... in these cases, the All-on-4 is the gold standard of treatment and provides an ideal functional and esthetic outcome.
If you’re only missing teeth and do not have a combination defect, then implant supported bridges are the ideal way to replace a full arch of missing teeth..... but understand, if your surgeon/dentist don’t understand this concept and attempt to use implant supported bridges in the face of a combination defect, then you’re going to end up with an outcome that will be less than ideal, will likely lack ideal esthetics, will be hard to keep clean and will have a higher risk of implant failure.
I perform a significant number of full arch implant cases.... while I use both treatment options, a higher percentage of patients are better served by the All-on-4.