Into LV6, both the old [BT6-111] Alphamon and the new [BT9-066] Alphamon are 3-cost to digivolve. If you have a starting memory of 3 (from Doumoto Kota) or generated some along the way (from a combination of Yuujis or Cool Boys), digivolving into Alphamon would probably still keep your turn. Otherwise, you would still be very safe staying at LV5 Dorugreymon or Dexdorugreymon thanks to the turn protection he has, or even better, the [Blocker] and [Retaliation] granted from Dexdorugamon.
Back to the Alphamons, both are great choices. However, in a more aggressive setup the new Alphamon does better, allowing you to retrieve an X-Antibody trait card from trash to be put under his digivolution cards. Ideally, an Ouryumon can be picked up if you plan to move into LV7 Alphamon Ouryuken, or another BT7 Dorumon if you wish for memory to keep your turn. Either way, you de-digivolve 1 of your opponent's Digimon.