There are lots of ways to build a gun-mage, and they differ drastically, so it's very hard to give suggestions without knowing more details about what kind of gunmage you're playing. Just to list off some possibilities: a Spellslinger/Sorcerer plays differently from a Spellslinger/Eldritch Archer plays differently from a Trench Fighter/ Spellslinger/Eldritch Knight plays differently from a Mysterious Stranger/Sorcerer/Eldritch Knight. There is a surprising wealth of support for the gun mage archetype!
Musket, pistol, and dragoon pistol are all viable weapons for different builds, and it really depends on your reloading solution. Dual wielding pretty much requires you to use the Juggler Bard archetype for your spellcasting. The problem is that you need a free hand to provide somatic components. If you're using a two-handed firearm it's a free action to hold it in one hand while you cast with the other, but if you're dual wielding there is no easy way to get the free hand to provide somatic spell components. Juggler bard can literally juggle firearms to give himself a free hand whenever needed.
Now, the question of reloading is the big one. Spell Cartridges is a good option for reloading, but it adds two more feats onto a build that's already going to be very feat-starved, plus it deprives you of your swift action economy which is a big deal at higher levels. However, there are other options; shadow-shooting weapons are a good one that frees up feats at the cost of a more expensive weapon, while if you've got the spell slots for it the reloading hands with dragoon cartridges can be a great reloading options. There are pros and cons to every approach, and I think you can make a good argument in favor of a wide variety of choices. Though do keep in mind that the Spellslinger/Sorcerer build doesn't need to bother with any reloading shenanigans; when casting spells through their arcane gun they don't need ammunition, and when not casting spells their firearm attack is so weak it's just not worth risking a misfire.
You don't need to invest 3 levels in musket master to use a musket. There are plenty of alternative ways to get free action reloading. Reloading Hands on Dragoon Cartridges, Spell Cartridges, and Shadowshooting weapons all work great.
For the Mysterious Stranger build, you want 1 level of Mysterious Stranger, 6 levels of Sorcerer and then the rest to Eldritch Knight. Presuming you take the Prestigious Spellcaster feat, that gives you 4th level spells and +6 BAB by 9th level which is a solid place for a gish to be. You can use the combination of Mending and Jury Rig spells to cover for the fact that you don't get Quick Clear. Jury Rig can suppress the broken condition if a misfire happens during combat, then you can cast Mending afterwards to fix it. I would probably go with a Shadowshooting weapon to avoid spending feats on reloading. By 9th level I'd aim to have Point Blank Shot, Precise Shot, Rapid Shot, Deadly Aim, Favored Prestige Class, and Prestigious Spellcaster. Alternately a human could drop Deadly Aim to take Arcane Strike and Spell Cartridges.
Well I Might do A Musket Magus (and Do spell Cartridges)
What would work better for a low to Middle level game
1: 1 level spellslinger and the Rest Magus
2: 1 level Gunslinger and the rest MagusI know both have there Advantages but i want to figure out what one i want and I'd like some comparisons to help me figure it out
Definitely Gunslinger, and while you're at you want to be human.
Your build will come online at 5th level as a Human Gunslinger 1 / Eldritch Archer 4. That lets you have (with human bonus feat) Point Blank Shot, Precise Shot, Arcane Strike, and Spell Cartridges. At 6th level with the Magus bonus feat you can pick up rapid shot, and at 7th level you take deadly aim, and at 8th level you get +6 BAB to get your first iterative.
With the Spellslinger approach everything is delayed since your BAB is one point lower and you need to pick up Quick Clear via Amateur Gunslinger so you're a feat behind. It's not the end of the world, but if you're mostly concerned about low-level performance the Gunslinger version is getting everything a little earlier.
Riving strike is a better choice than rapid shot. Shoot once to lower saves, cast your spell, hit then with your spellstriked shot. Since you are already committed to taking arcane strike Riving strike is a really good choice for a magus.
Get a Large-sized Double Hackbut and carry it around in your pocket with Shrink Item. Then w/e you need to bring the pain, end the Shrink Item spell and presto, instant cannonade :)