Reflect is a better option than Withdraw to gain armor.
Is it though, really? If Reflect is a "better" option because it raises defense faster ... it's a pretty minuscule difference.
Tauros Body Slam vs. +1 Withdraw Slowbro: 58-69 (14.7 - 17.5%) -- possible 6HKO
Tauros Body Slam vs. Slowbro through Reflect: 44-52 (11.1 - 13.2%) -- possible 8HKO
That extra ~3-4% damage decrease because you used Reflect instead is virtually meaningless in a practical sense.
Tauros Hyper Beam vs. +1 Withdraw Slowbro: 102-120 (25.9 - 30.5%) -- guaranteed 4HKO
Tauros Hyper Beam vs. Slowbro through Reflect: 76-90 (19.3 - 22.9%) -- possible 5HKO
~5-7% ... obviously a little better, but still nothing to write home about.
Snorlax Body Slam vs. +1 Withdraw Slowbro: 62-73 (15.7 - 18.5%) -- possible 6HKO
Snorlax Body Slam vs. Slowbro through Reflect: 46-55 (11.7 - 13.9%) -- possible 8HKO
~4-5%
Snorlax Hyper Beam vs. +1 Withdraw Slowbro: 108-127 (27.4 - 32.3%) -- guaranteed 4HKO
Snorlax Hyper Beam vs. Slowbro through Reflect: 81-96 (20.6 - 24.4%) -- guaranteed 5HKO
~7-8%
Snorlax Self-Destruct vs. +1 Withdraw Slowbro: 188-222 (47.8 - 56.4%) -- 88% chance to 2HKO
Snorlax Self-Destruct vs. Slowbro through Reflect: 141-166 (35.8 - 42.2%) -- guaranteed 3HKO
~12-14%. This one is more impactful, but it all depends on how much you were chipped and what has the potential to revenge kill you anyway - so that's pretty situational.
Long story short, looking at the actual math on this, in practice that small boost in defense faster doesn't look like it's going to make any kind of difference in many match-ups. In an age where PP stalling out Rest Snorlaxes and Reflect Chanseys is getting more and more common, I'd rather have more PP, more max armor (up to +6), and a speed dropping effect by using Withdraw than having Reflect for practically negligible increased protection in 1 turn in exchange for a lower armor ceiling, fewer PP, and no speed dropping effect.