Ahh so you mean do one or the other, not both..
Hmmm, good question.. A guess a little of both. (Not a very definitive answer, but there you have it.)
Solo time is very valuable, as you learn to deal with ATC by yourself (Always get flight following when you go on X-Cs! Not only because of the increased safety, but because it gets you used to working with ATC) and you will have a greater understanding for why you have to try to be the best pilot you can; If something goes wrong up there you only have one person to trust; Yourself.
But some basic instrument skills also helps a little. If it's something as simple as tracking and navigating by the use of VORs.
A PC based flight sim isn't much good when it comes to learning how to fly an airplane, but it can teach you quite a bit about navigation and VORs and so on, so if you have a PC and can get hold of X-Plane (Or MS FlightSim, if you feel the need to follow the rest of the sheep..
) play around with that. There's several 'do it yourself' VOR/Nav training web-pages on the net, and although none of this will make you a instrument expert it certainly helps to have some understanding about all of this when you start the program.