I am going on a group trip to Japan. I want to extend on the front end. If we are spending 2 full days in Tokyo should we extend for more tone there or go to Seoul?

FWIW, I wish I'd had more than 4 full (very full!) days in Tokyo, not counting time in places that many people visit as day trips from Tokyo. I was glad to have a full week in Seoul.

But that would seem beside the point: Only YOU can decide what YOU want to see and experience. If it helps, japan-guide.com is an excellent resource for all travel information for Japan. For Seoul, you might find some useful information in my old trip report:
4 wonderful solo weeks in South Korea

Moved from Tech Support to the Asia forum and tagged for Japan

Two full days is not enough for Tokyo. That would barely scratch the surface, but maybe that is all you want/need (as a tour-taker).

If you were to go to Seoul then how many days would you spend there? I guess that would mean visiting Seoul first and then flying to Tokyo to meet your tour. Would that be your plan?

When would this trip be? How much time in Japan for the tour? Please provide a link to your tour itinerary.

I am taking "group trip" to mean a tour.

I have 4 full days in Tokyo and it was not enough.

FWIW, I prefer Busan to Seoul. In that case I would be tempted to go there. If you can only go to one place in Korea, Busan has got a little bit of everything. Mountains that plunge into the sea. A rare seaside temple. A rare temple built into a cliff. Vast sections of the city with traditional markets.

Since you mention extending on the front end of the trip, recognize that you will be pretty wiped out due to travel and time difference....I would vote for adding the days in Tokyo and acclimating to everything, especially if the group tour starts in Tokyo. that will give you time to settle in, explore a little bit on your own and hit the ground running, so to speak, when the tour begins. also, factor in the travel and hassle time of travel between Seoul or Busan to Japan, and having to go through customs and immigration in both countries.

I think it depends on how much front end time you would have. For my part, I'd want a minimum of four days in Korea (preferably at least a week) before heading to Japan. Shorter than that, I'd prefer to minimize the time spent traveling and just spend more time in Tokyo. Of course, we're seniors, so extra travel might not be as big an issue for you.

You should definitely have more time in Tokyo.
it has a lot to see.
Moreover, a few months and years later, all those places which you saw in a jiffy will be erased from your memory.
you will only remember good experiences that you had while exploring slowly. Thats the real bang for your time and bucks.

Korea has many other places to see beyond Seol. Make another trip and explore them well

I've found that people who spend just a couple days in Tokyo are often overwhelmed by it, largely because they spend those couple days at the busiest and most popular sites. Adding more time there will allow you to wander through neighborhoods (Yanaka is great for this), visit parks and gardens (the Nezu Museum garden is beautiful), people watch from a cafe, and/or spend hours picking out pens at Itoya!

The OP has not responded. But, I have had this thought to add:

You are going to go to Tokyo for the tour. Extra days there are free! There are no additional costs of time, money or effort (unlike adding Seoul).

The Nezu Museum suggested by memejs is a real treasure. The exhibits themselves can be hit or miss depending on your interests, but the gardens are exquisite in the middle of busy Tokyo. Highly recommended and close to popular Omotesando and Harajuku with a totally different vibe.