I've gamed the War of 1870 both with 15mm and 28mm figures using two different rule systems. The first thing you should ask yourself is what do I want to accomplish when I game this period. Are you happier painting up and gaming with 28mm figures at a more tactical level? Or do you want to save money and just stick with 15mm figures?
The cheaper way is to do this period in 15mm. You still have options with rules. There are some Fire and Fury variants for the Franco-Prussian War using some old Wargames Illustrated issue from the 1990's. I think they might be available online as part of a Yahoo Users Group. But I am not sure. The first set of rules I played about 12 years ago was They Died For Glory by Dave Waxtel and Bob Burke. Unit are organized by battalion, battery, and cavalry regiment. This rule system is designed with a mininium of a French infantry division and a divison and a half of German infantry. I had an entire 15mm 1870 French infantry division, 1 15mm 1870 French cavalry brigade, 1 15mm 1870 Prussian infantry brigade, 1 15mm 1870 Bavarian infantry brigade, 1 15mm 1870 Wurttemburger infantry brigade, and 1 15mm 1870 Prussian cavalry brigade. That worked out to a corps of 1870 German infantry. Figure scale is 1:50.
They Died For Glory is a good, solid rule system. It works with a French and German movement phase. While one side is moving the other side can respond with movement or firing. Morale is rather simple using a 1D6 system. The French infantry fire is particularly deadly. So is the German artillery fire. The battles aren't elegant affairs. It is usually a matter of bringing a line of skirmishers and then launching your Prussian infantry columns headlong into the French infantry lines once they have been softened by artillery fire. Most of the terrain is meant to be on the French side of the board. And the French are outnumbered to the Prussians at least 1:1.5. Cavalry can be used to launch suicidal changes to slow up advancing infantry columns, the only use they have is for attacking limbered artillery. The thing I don't like about They Died For Glory is the combat system.
You have to figure out how many dice you get in combat depending on the nationality and unit type of troops. Prussian dice to figure ratios are much higher than the French. And the Prussians normally roll over them in close combat. The problem with They Died For Glory is that the pick-up games are rather predictable. It usually a matter of stopping German infantry advances and hiding behind fortified positions.
That is the same lesson for Chassepot and Needlegun. This system is by Larry V. Brom. It is designed for 28mm figures. It can accomodate anything from 25mm down to 6mm scale. This is a more tactical set of rules. There is random unit activation and movement. This rule system is designed in the same vein as The Sword and The Flame. There is a movement, artillery, small arms, close combat, and morale phase. The largest unit is designed for an infantry division. French infantry units are smaller than German infantry units. Moral is determined randomly for each basic unit when it first needs to test it. That is something different than most TS&TF variants. Chassepot and Needlegun is a very friendly convention style wargame that's easy to play. They Died For Glory isn't. They Died For Glory requires a good GM. Chassepot and Needlegun doesn't. If left to my own devices, I'd play Chassepot and Needlegun. Though it requires more figures, I don't mind it. You get a game done faster with Chassepot and Needlegun than with They Died For Glory. Chassepot and Needlegun also have variants for Crimean and Seven Weeks War of 1866.
There is also Black Powder. I don't have a copy of the rules. It's designed for this period. You can use whatever scale of figures you want. No doubt it would work well for 1870. And there is also 1870 (The Rule System), too. Again, I haven't played it. So I can offer no opinion upon it other than what I've read about. If you are going to game 15mm FPW, my figure recommendations are Rank and File and Essex Miniatures. If you want to do this period in 28mm, you have to use different manufacturers. My recommendations are Wargames Foundry, Battle Honors, Askari Miniatures, Old Glory Miniatures, Castaway Arts, and North Star Miniatures. Unfortunately, there is no one who is doing 28mm plastics right now. I'd definitely buy more figures if Perry Miniatures decided to expand into this period, too.