Tire selection is a huge factor too, as well as braking ability.
On a 15 or 16" wheel, the biggest brakes you can go are the Mustang GT PBR calipers, which are a HUGE improvement over the stock calipers, but still a limiting factor as you are stuck with stock sized rotors. Also, tire selection in 15 to 16 is very limited. You can get 'stock sized" tires (225/60/16) all day long because they are a very common, pedestrian tire, but getting wider, better performing tires on 15 to 16 inch wheels, you have limited selection. Add to it the tall sidewalls, and you can get much better cornering without much decrease in ride quality with 17 and 18" wheels.
I'd honestly go with 18's for a "fun" street car. On my car, with 255/40/18 on 18x9" wheels with 35mm offset, I don't rub, I have 89SC sway bars, Eibach lowering springs (which are softer than the Vogtland springs), Shock Connectors and my car still rides very well. Yes, it's marginally stiffer than stock, but it corners much better, has much better overall traction, and there is a HUGE selection of tires to choose from. 17 and 18" wheels also allow for the relatively inexpensive upgrade to 13" "Cobra" brakes, you can get rotors/pads/calipers all day long from Rock Auto, which offer yet again a big braking improvement over the PBR calipers. Or if you want to go big, you can get some nice Brembo or Wilwood calipers and rotors.
97 Thunderbird LX (The GT Bird)
2003 Explorer WAP block with Modular Head Shop "street ported" heads and Stage 2 PI NA cams, 75mm Accufab throttle body, C&L upper intake plenum, Kooks 1.75" primary/3" collector headers, 2.5" full exhaust with mid mount Magnaflow dual in/out muffler, 24lb/hr injectors, 80mm MAF, Tuning from Don @ www.lasotaracing.com
, CAI that feeds from fenderwell. Jmod, 3.73:1 TL in Mark VIII carrier, Mark VIII aluminum LCAs, 93 Mark VIII driveshaft, PBR brakes (soon to be Cobras), 18x9 wheels with 35mm offset, 255/40/ZR18 Tires, Front and Rear strut/shock bracing, GR-2 shocks, Eibach 1.5" springs, 1989 SC front and rear sway bars.