Combat was a lot more deadly under FNFF, and 2020 rules, so the players tried to find a way to avoid combat. Just wish I could have found a way to integrate Heavy Metal into the system.
I felt that combat was pretty much about the same level of deadliness in both systems.
Players and GMs have always had difficulty dealing with armor since the days of Dungeons and Dragons. I didn't feel that 2300 or 2020 were any different in that respect: Combat is very deadly if you don't have body armor. If you do have body armor, throw the pistols out.
The problem is that CP's AV12 (which isn't hard to get - skinweave) from Cyberpunk essentially made you immune to anything short of a 4D6 pistol. People could slot AP ammo, but the damage halving mechanism plus BTS meant your players were only ever taking 1 point of damage. Even if you didn't allow skinweave, again, it's not hard to get enough armor to have AV12 over your entire body except your head (because helmets look stupid). With armor layering rules in CP 2020, it's not difficult to get AV20+ over much of your body, again save your head. Once you hit AV20, even 5D6 assault rifles are basically "1 point attacks" with AP and not AP making no difference after BTS. Every runs around 6D6+1 assault rifles as result.
The mirror problem was armor in 2300. In that sense, 2300 was deadlier since 2300's armor didn't have the emphasis on being stylish or at least inconspicuous unless you used Earth/Cybertech's subdermal armor. Hence, players wore less armor or no armor. Because of the strangely long ranges of 2300 weapons, players spent most firefights in the "double DPV" ranges. While this meant nothing with unarmored players and rifles, it made the 0.3 and 0.4 pistols deadly as they were now 0.6 and 0.8 DPV weapons. A roll of 1 or 2 meant the players had a good chance of being autokilled in 2300 (20% of the time), whereas in CP the autokill range was just the head (10%).
I'd really say that both systems had issues with being unable to miss with your bullets. At short combat range, the target numbers meant that even a person with very basic weapons skills had a better than 50% chance of hitting with any shot. In that sense, 2020 was less deadly as it was more difficult to hit with your shots. However, once all the Solos start piling up the cybernetics, getting their equipment bonuses for +5s and +6s to hit on top of their native skill, again, they found it so difficult to miss they were doing called shots on the head or in my group, the infamous "run my full movement rate to pointblank range then do a called shot on the head at -3 for multiple actions" trick.
Missiles in 2300 were far, far, far deadlier than equivalent weapons in 2020. The Homing Value system was flawed. Since most vehicles that players would ever get had no countermeasures, missiles were fired and simply rolled the HV, which again, meant they rarely ever missed. Against a lightly armored civilian vehicle, even an obsolete missile would tend to send it straight to "vehicle destroyed" ... along with all the passengers. This level of deadliness in CP only came out with Maximum Metal where they gave you some benefits for the d10 damage weapons (which up to then had been laughable, as the 4d10 B-A sniper rifle had an inferior minimum and average damage to a 6D6+1 assault rifle).
I really wanted to try 2300 using Millenium's End rules. Sadly, some of my players could never get their heads around the idea of "hitting but still missing" - getting the target number but then missing because of the hit location on the bodymap, so I never had a chance to try it.