I need help on deciding which to use, anyone that can help would be greatly appreciated.

This gengar:

Gengar@Leftovers
-Lonely (I think its lonely)
-Substitute
-Focus Punch
-Thunderbolt
-Shadow Ball

Or

Gengar@Petaya Berry/Leftovers
Timid:
Giga Drain/Will-o-wisp
Thunderbolt
Psychic
HP Dark/Destiny Bond

Please tell me which is generally better in competitive battling, thanks.

First of all, you can and should post such stuff in The Rating Center

However, since you mentioned competitive battling, the rating center you should go to is the Netbattle Team Rating Thread

Now let's talk about your Gengars. The second one has a better Nature for sweeping (Timid = +Spd -Atk) compared to the first one (Lonely = +Atk -Def). So I would personally use the second one. Besides, Gengar has good Special Attack, and can pull off the moveset well. Destiny Bond over HP Dark, while both Giga Drain and Will-o-wisp are fine.

The 2nd is better and had it held leftovers. A pretty good moveset
Just use HP Dark over Destiny Bond as it would be a good move against
ghost types. And perharps add a fighting move like Dynamic Punch.

i would go for the second but it would be a bit better with the leftovers on it, No need for petaya berry, cos u ent gunna want your attack to rise while you dieing, u would want one that raises sp.atk,
(i dunt no if there is a berry like that tho)