I use to own a GTA 500 - a great machine - loved it - until I bought a Tesoro. Never owned a 250 - but I saw one in action once. The 250 was lighter and I think it did a slightly better job at identifying targets. The 250 is more advanced, but if your 500 is working fine, then why switch? It's a deep machine. Don't know if the 250 is a deeper machine than the 500, but those old 500's went deep. (I would think a 1350 would be more of a step-up than a 250.)
If you want a really fun machine that goes deep, loves silver, has a fast recovery time and is really easy to pinpoint with, try a Tesoro Umax Silver. It doesn't have a display, but the discrimination is excellent and you can swing for about 30 hours on one 9-volt battery. Very light-weight - you can swing it all day. (I missed the display at first, but after a few hours, I didn't miss it anymore. (Never could trust the display 100% of the time anyway.) Now, instead of keeping my eyes on the display, I get to look around and enjoy my surrounding.)
HH
Rich
P.S. Welcome to the forum!