Really a question for the folks at Team Titliest but many times the only difference is cosmetic.  Maybe a paint or printing flaw that doesn't pass inspection.

One say's "practice" and the other one does not.

There is no difference. It is just like putting a logo on your ball.

Praise the Lord!

I guess the next question would be is regarding the legality of the "practice"... is it legal for tournament play (amateurs)? minor tournaments such as outings and league nights?

how about when one scores a hole-in-one using a ProV1 practice??? and using it for rounds for handicapping??? just wondering...

Q. What is the status of "X-out," "refurbished" and "practice" balls?

A. "X-out" is the common name used for a golf ball that a manufacturer considers to be imperfect (usually for aesthetic reasons only, e.g., paint or printing errors) and, therefore, has crossed out the brand name. A "refurbished" golf ball is a second-hand ball that has been cleaned and stamped as "refurbished."

In the absence of strong evidence to suggest that an "X-out" or "refurbished" ball does not conform to the Rules, it is permissible for such a ball to be used. However, in a competition where the Committee has adopted the condition that the ball the player plays must be named on the List of Conforming Golf Balls (see Note to Rule 5-1), such a ball may not be used, even if the ball in question (without the X's or without the "refurbished" stamp) does appear on the List.

In most cases, "practice" balls are simply listed, conforming golf balls that have been stamped "Practice," in the same way that golf balls often feature a club or company logo. Such balls may be used even where the Committee has adopted the condition that the ball the player plays must be named on the List of Conforming Golf Balls.

(http://www.usga.org/Rule-Books/Rules-of-Golf/Decision-05/#5-1/3) 

Thanks for the info Mike... Anyway, I've basically committed myself on using brand new ProV1x's from now on. One less thing to worry about rules. And sometimes, it is embarrassing to use balls with x-outs stamped on them... hehe 

 If this is case, wouldn't it be smarter for everyone just to buy the Practices? BTW are practice balls the same as X Outs

To that end, only products that are of a minimal defect are available for purchase. These products are available in two offerings:

1. Pro V1 Practice golf balls are conforming products that differ only due to a cosmetic blemish such as paint, ink or registration of stamping. Pro V1 Practice golf balls do not have any construction or performance deficiencies.

2. Pro V1 X-Out golf balls are also conforming products. However, they usually have a cosmetic blemish and/or occasionally have a minor physical defect that should not significantly affect its performance. More often than not, the physical condition causing the ball to be stamped as an X-Out is so minute that it is not recognizable by the player.

One thing to keep in mind, both products may be used in the casual round of golf including those with scores posted for handicaps, and most competitions, with the possible exception of high level competitions invoking the Local Rule requiring balls to be on the List of Conforming Golf Balls. Please refer to the USGA Official Rules here: http://www.usga.org/Rule-Books/Rules-of-Golf/Decision-05/#5-1/3

As a result of the potential for an X-Out to be non-conforming if tested, we do not recommend using this model during tournament play. 

Do they sell Pro V1X practice golf balls?

Yes, they sell both, ProV1 and ProV1x practice.

When you buy a dozen, they normally do 2 sleeves of ProV1, and 2 sleeves of ProV1x. I'm lucky enough that my store lets me swap out two sleeves and get all 4 sleeves of the same ball.. Must be another guy that likes the ProV1x Practice only in his dozen!

I play the new ones during the spring and summer. I play the practice in the fall when I can lose them in the rough and leaves.