These models r closely related.
The differences:
The Slayer uses duckbill ASA, plastic grip frame, black grips, LED display & less milling on the body.
The Wrath uses a non-standard dovetail ASA, metal frame, clear grips, & LCD display (thus clear grips to see LCD).
The similarities:
Same barrel, feed neck, regulator, body (except the Wrath has some additional milling which is negligible to performance).
If u r choosing between the two - I'd opt for the Wrath since it has the LCD display & metal trigger frame.
Upgrades will depend on the Wrath that u get & personal preference but commonly done r bolt & board for higher ROF.
Bolt: Delrin or Teflon bolt & pin if u get the marker with the aluminum bolt. You can get these online from Alamo City Paintball. If u get a final generation or Wrath LTD then it has a plastic bolt with is comparable to Delrin.
Boards: If u get the BD5 board, it is okay but hard to reach past 10bps. You will probably need to look at the Scenario Dreams T-Board for about $60 or Virtue for about $135 or get the R15 board from Kee Action Sports (will work with stock LCD unlike aftermarket boards) for about $65. There is also the Lucky Board but that is pretty rare to find. Stock BD5+ will be better but some still upgrade from that one to the aftermarket ones listed above. If u get the R15 off the bat, you're good since it is as good as the aftermarket boards. FYI - Stock board model will display on LCD when first turned on.
Barrel: Autococker threads