Before deciding on the tent to buy think about the characteristics of your different options.
- Tent size (how many people does it hold, is there room for luggage or even indoor cooking?)
- Tent weight (you want to keep that as low as possible obviously)
- Type of construction (generally this doesn't matter too much unless you have a favorite)
In terms of weight-per-sleeping-space you can generally say that the bigger the tent, the more efficient it is. So while a one person tent is slightly lighter overall, a two person tent is a lot lighter when the load is shared between two people (or as a probably more realistic example: one person will carry the tent, and the other the cooking equipment). And again a three person tent will be more efficient for three people, etc.
The one person tent
Will have a slightly lower weight generally, but much less space. I'd say that you are generally saving at best 20-30% of the weight as compared to a two person tent, but have something like 50% less space.
The two person tent
Will generally be a bit heavier, but will also provide much more space. Not only can you bring a friend along in the future, but if you are using it alone it will also be a lot more comfortable, there will be room for the backpack, to get dressed inside, etc...
To sum it up:
Unless weight is a very critical factor that you need to minimize absolutely - for example when doing very long (10 day+ hikes) - I would go for the two person tent. It is more versatile, more comfortable and the weight savings of the one person tent are just not worth it.