I take it you are matari and that you mean auto cannons rather than Gallente Rail guns?!
This depends on what you do, if you are looking at mainly PVE or being a sniping specialist, ie going for Sniping HACS or Tempest, you are best off with the Alpha and Range of Arty.
If however you are mainly PVP and are looking to go for main fleet ships such as Battlecruisers/Battleships and general close range ships such as Dictors, Hictors, Inty's etc you are much better off with Tech 2 AutoCannons which pump out more DPS.
Also, if you want range you can get respectable range in fall off with Barrage.
If you do both PVE and PVP I would still lean towards AutoCannons and then do large & medium T2 Arty afterwards...
You shouldn't lose PVE ships very often if at all, so you can afford to invest in Scout versions of the arty which are nearly as efficient as T2 just without the ability to use T2 ammo.
Minmatar have both a blessing or a curse, we have so much flexibility on ship layout and using the right tool for the job is important.
Problem is that means 2 times as much training! You will eventually be able to get both, but if you are still struggling to decide, consider this.....
Medium guns go on Battlecruisers and Cruisers and smalls on Destroyers and Frigates.
Large guns only go on battleships!
If you are happy not rushing to battleships and would rather consolidate your skills with the smaller ships, getting T2 arty mediums and T2 autocannons in smalls and mediums will serve you really well...
The Hurricane is an excellent passive shield tanking 650mm artillary platform as well as a potent PVP platform when armour tanked and fitted with 180mm or 220mm autocannons.
Rifter is excellent with 150mm gattlings, and cruiser wise the rupture is a miniture nightmare!
this will offer you the most flexibility then when you are ready to graduate to HACS you won't need new Arty skills, and if you are looking to go battleships then you can again asses sniping fit of the tempest or the autocannon and torp fits of the typhoon.
Ahh don't completley neglect missiles, you will use them later in your career!
One last thing, mixing Arty and Auto cannons in fits is a big NO NO, you make yourself Jack of all trades and master of none, given same skills and same ship as your opponent you will loose at close range to an autocannon fit because you won't have the DPS for a knife fight and you will loose at long range because you won't have the Alpha for a sniping contest. If you run out of grid or cpu, better taking the calibre down a notch (ie from 720mm's to 650mm's) rather than mixing weapon types.
Trying to use same calibre guns is also a good idea as they will then have same optimal and fall off/ damage potential.
Only time mixing weapon types is acceptable is missiles and turrets on matari ships, and then I would always have a primary and secondary and try to keep ranges similar.... ie close range for Autocannons and long range for Arty.
What ships are you flying? That is the most important factor in answering your question…