Should dual classed Assassins put points in stealth, or should they rely on invisibility?

Hello everybody! I should probably preface this by saying that despite me putting this in the bg1 category, it will also affect bg2. I haven't played the full saga in a few months, so I decided it was time for a new run. This time around I wanted to play a class I've never played before, being an assassin 13 dual to cleric. Should I put all of the points I get from assassin into the stealth skills or should I focus on other thieving skills? I'm not sure what other party members I'm going to use, although I could probably just use Monty in the first game and then Jan in the second for any other thieving skills I may need. Despite me having thousands of hours in these games I've never actually played an assassin before, so thank you for any recommendations!
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So, I think you have to think hard about your party composition for BG1 and even BG2. Are you going to take another thief, say Jan in BG2? Then you can focus purely on stealth.
That being said, at the later stages of the saga, using invisibility spells, potions, abilities is often better than stealth, especially if you're aiming to score regular backstabs. Even by the second half of BG1, where you have access to more shops, an invisibility ring (albeit quite pricey), invisibility ends up working more practically than stealth. So you certainly can build an effective assassin, one that takes advantage of backstabs while putting no investment into stealth.
.....True, you won't have a lot of spells as a low level mage, but I assume you will have party members to carry you through those awkward first few levels. Just seems like a better fit, skill-wise. But maybe you have a specific reason for preferring a cleric? Planning on using some uber weapon like the +5 Flail of Ages, perhaps? But you can't get that puppy fully upgraded until half-way through ToB.
Now, as for Non-Detection ... the spell has problems. While it does block Detect Invisibility and Invisibility Purge, it's useless against the higher-level divination attacks. If an invisible character with Non-Detection up gets hit by Oracle or True Seeing/True Sight, the spell both removes the Non-Detection and reveals the character. You see, ND counts as an illusory protection, which those spells remove. And they do that removal before the part of the spells that reveal invisible foes.
Item-based non-detection, like the cloak, does not suffer from this problem. If you're stealthed with a cloak of non-detection, nothing can reveal you. The only counter to that is enemies that see through invisibility.
Well, doh! Obviously they don't need anything else. But I was curious if the poster did in fact have a more specific reason. For example, maybe his group already has a mage and someone with raise dead ability would be crucial down the road. No harm in asking.