Solo or Group?

During Christmas Break, my husband and I have managed to find some time to play SWTOR together, he on his bounty hunter and I on my 2nd agent, Kaminoko.  It's been a lot of fun, being able to run around places together, although with Mako coming along, the only times I get to heal are when we're facing gold-level opponents.  (Incidentally, right now, Kaminoko and Mako look like they are wearing uniforms or something . . . Somehow they managed to get chest pieces which are all but identical-looking.)

I really only have one complaint:  everything is falling over a bit too quickly.  Except when we tackle an Heroic 4 situation, there is little to no challenge.  Because we do take the time to run the Heroic 4's and because it's so easy to make normal groupings fall over that we never really avoid doing so, we've leveled so quickly that we're already level 23 after finishing only the first sector on Nar Shaddaa.

With my first agent, who believed a good agent inflicts as small a body count as possible, I remember being finished with Nar Shaddaa and moving on to Tatooine by this level, grumbling because I was on foot, having to traverse vast tracts of land in Mos Ila.  (Remember, back then, you had to be level 25 to use a speeder at all.)  And instead of steamrolling encounters, I had to carefully plan, sneak, control crowds, and so forth.  When I completed a challenging quest, I felt I had really accomplished something.

So here's the question:  is it more fun to level playing solo or playing in a group?  I wouldn't trade playing with my husband, but I do have to say I found it more personally satisfying soloing my way up, because it was much more of a challenge.

What do you think?  Solo leveling, or leveling with a partner?