I'll just give you my opinion and experience..
Keep the 503 !! , it's a bit more clumbsy to work with, than the modern 645, but the build quality is SOOOOOO much better..
I made the mistake to buy the Phase/mamiya body when going MF, and every day i am affraid it will break on me..
That's why i got a Technical camera ( Arca Rm3di), i was looking to the Linhof techno to.
But with some help of LM forums, an other owner, AND the quite "cold" reception of the Linhof people at Photokina, i went for the Arca, ...
As it also has a sliding back but it's less flimsy ..
Also, Watch out for focussing digital wide angles on the Linhof, you'll need some practice.. As from 35mm / 47mm on it's quasi impossible to see what you're doing.. you'll have to work blind..
The thing is , if you don't need wide angles concider the Linhof, Only, a 679 or Arca M or F is much more precise ( you also have 2 moving planes instead of 1 on the techno)...
If i could do it again ( and i think i'll swap my Phase One 645 back soon for a Hasselblad V mount soon), ( whatever back you choose), keep your Hassy 503cx, back to the good old quality, and maybe find a bigge magnification to see what you're doing..
OK you'll need a synchro cable, but i prefere that over not knowing i can shoot;..
I have also loads of colegue/friends, with the same problem with the 645 (mamiya or hassy), when using them hard they will fail much sooner than the older hassies.. ( i even have 1 friend with a "large" commercial studio, sending in bodies weekly for repair, he has 12 in rotation, 4 in repaire, and 2 on shooting reserve, using 6 on the studio..)
ah and the hasselblad tilt shift solution is "OK" , but a good old fashion technical camera ultimately gives you much more...
It's a choice, the hassy solution isn't bad at all, but in architecture you'll block soon, just go for a good technical camera with the apropriate lenses, and some practice..
Again Watch out for focussing the techno and wide angles, try renting a "pancake" camera, arca, alpa, cambo, sinar, etc etc.. you can focus blind on those, and it's a big help..