It is likely likely to be quite warm after a couple of days. Both ships are suitable for ocean crossings but as Ventura is bigger, it will be more stable. We did a crossing on Oceana two years ago and had no problems. However you can get storms anytime anywhere.
If the itinerary of the ship you choose is taking the more southerly route that is from, say, the Canaries to the Caribbean then you stand a good chance of warm to hot weather and a reasonably smooth crossing. If, however, it is from the UK to North America, like New York or Boston then the chances are that it will not be the same and this October for instance saw some really bad storms across the N. Atlantic. I could make a fortune if I could predict Atlantic weather so nothing is stated in the absolute knowledge of what to expect. I always choose a more southerly route when options allow.....Nei.