I did some research on the exact ratio of each gear, so you know. It doesn't say the exact speed, but it does give you an indication of what the acceleration will be like. That way you can make your own decision.
The first number is the number of teeth on the gear before it, the second is the number of teeth on that gear. So, 39/15 will be 15 teeth coming from 39 teeth. Then I have the speed range of each gear. The first number is the speed at 1000RPMs, or the speed the bike will travel when you just let go of the brake(of course, as we all know, the bike will most likely stall out after 1st or 2nd gear at 1000 RPMs at a standstill, so the second number is 3,000 RPMs, the RPMs you drop to when picking up a new gear. The third is the max speed at each gear. (My source is unsure as to the max speed since it varies from bike to bike, plus going faster may damage the bike, because you enter the red zone in terms of RPM.). As you know, larger to smaller=more torque but the engine works harder. smaller to larger makes the engine not work as hard(like on a bike with multiple speeds big disk is easier to pedal)
Here is the kawasaki ninja 250r 2008
Gear ratios: 1st- 39/15 (2.8/9.1/34.3)
Gear ratios: 2nd- 34/19 (4.1/13.2/49.8)
Gear ratios: 3rd- 31/22 (5.1/16.8/63.3)
Gear ratios: 4th- 29/25 (6.2/20.4/76.9)
Gear ratios: 5th- 27/27 (7.2/23.6/89.1)
Gear ratios: 6th- 25/28 (8.1/26.5/100+)
As you can see, 90mph maxes out 5th, in 6th it is 11,000 RPMs. 80mph is 11,000 in 5th and 10,000 in 6th. Both are before the red zone.
Kawasaki ninja 650r(aka. ER-6) 2006-2008
Gear ratios: 1st- 39/16(4.7/15.2/9.7)
Gear ratios: 2nd- 36/21(6.6/21.6/56.4)
Gear ratios: 3rd- 32/ 24(8.5/27.7/72.5)
Gear ratios: 4th- 30/27(10.2/33.3/87)
Gear ratios: 5th- 28/29(11.8/38.3/100.1)
Gear ratios: 6th- 23/27(13.3/43.4/113+)
**please note, the stats for the 650r only go up to 9,000, so the speed is much higher in each gear if the bike goes past 9k. However, it may be due to the 650 having a lower RPM range for more torque.
As you can see, the 250r will have less of a chance at stalling out in slow moving traffic than the 650r. And if it is true that the 650 only revs to 9k then you only see 10mph difference in top speed.
Anyway, those are my deductions. You can make your own based off of that info.