I currently own an AT PRO and used a GTI 1500 for 4 years. I also have a White's DFX and a Fisher F75 along with the AT PRO as the machines I currently use.
The first thing I will say is don't expect any additional depth, target ID accuracy, or recovery speed from any of the units you mentioned as possibilities. Your Tek Omega is already as good or better as any of the other models you mention. If you are a coin shooter than the Imaging feature on the GTI series machines might be worthwhile but the GTI 1500 is based on 15 year old technology and isn't as deep or as fast as your Omega which is based on a 2007 (I think it is) design.
The AT PRO might be a good choice if you want to hunt in water environments. A good environmental cover will protect your Omega well enough against rain and will also keep the display plate from getting scratched so the AT PRO isn't really going to give you anything you can't already do with your Omega in the rain. The AT PRO is a brand new design and although I haven't had a chance to take it relic hunting due to the deep snow cover here in NH it seems to do ok at salt water beaches and it is waterproof to 10 feet. One thing you should consider is that it uses waterproof headphone connectors so you have to use the supplied headphones or buy the water proof headphones from Garrett and if you have any high quality detector headphones (such as Grey Ghost, Killer B's, or Sunrays) you won't be able to use them with the AT PRO. The machine is also poorly balanced and so will feel clumsy to swing next to your perfectly balanced Omega.
The F70 is made by the same company that makes your Omega so it will seem very similar to you, however if you buy the 11" DD coil for the F70 (assuming you don't already have one on your Omega) than you might get a little better performance from the F70 as it is based on the T2 platform while the Omega is based on the BH Platinum/Fisher F5 platform so they do have different designs with the F70 being based on First Texas' top performance platform, while the Omega is based on their mid-line platform. Also with the F70 operating at 13Khz instead 6.5Khz (or something like that) like your Omega does, it should perform a bit better on lower conductivity targets.
All and all if I was limited to choosing from among those 3 machines with the idea of keeping the Omega I would probably choose the AT PRO and buy a $30 Garrett Gizmo off of Ebay to help lessen the balance issue and also to extend the back rod of the AT PRO so it would be less uncomfortable to use.
A few other ideas you might want to consider though is getting a machine that will be operating more differently than your Omega such as a multi-frequency machine. For example you should be able to pick up a used Minelab FBS machine such as a Explorer 2, or a Safari for that price range, or perhaps even an XTerra 705 as you can change its frequency by changing coils and so it can operate at 3 Khz, 6.75 Khz, or 18.75 Khz by changing coils.