都是打包一堆信号，区别可从源码中看出：

+ (instancetype)zip:(id<NSFastEnumeration>)streams {
    return [[self join:streams block:^(RACStream *left, RACStream *right) {
        return [left zipWith:right];
    }] setNameWithFormat:@"+zip: %@", streams];
}

+ (instancetype)zip:(id<NSFastEnumeration>)streams reduce:(id (^)())reduceBlock {
    NSCParameterAssert(reduceBlock != nil);

    RACStream *result = [self zip:streams];

    // Although we assert this condition above, older versions of this method
    // supported this argument being nil. Avoid crashing Release builds of
    // apps that depended on that.
    if (reduceBlock != nil) result = [result reduceEach:reduceBlock];

    return [result setNameWithFormat:@"+zip: %@ reduce:", streams];
}

+ (RACSignal *)combineLatest:(id<NSFastEnumeration>)signals {
    return [[self join:signals block:^(RACSignal *left, RACSignal *right) {
        return [left combineLatestWith:right];
    }] setNameWithFormat:@"+combineLatest: %@", signals];
}

+ (RACSignal *)combineLatest:(id<NSFastEnumeration>)signals reduce:(id (^)())reduceBlock {
    NSCParameterAssert(reduceBlock != nil);

    RACSignal *result = [self combineLatest:signals];

    // Although we assert this condition above, older versions of this method
    // supported this argument being nil. Avoid crashing Release builds of
    // apps that depended on that.
    if (reduceBlock != nil) result = [result reduceEach:reduceBlock];

    return [result setNameWithFormat:@"+combineLatest: %@ reduce:", signals];
}

可见区别主要在return，则可以把焦点放到combineLatestWith:和zipWith:两个方法中

return [left combineLatestWith:right];
return [left zipWith:right];

zipwith方法的源码：

- (RACSignal *)zipWith:(RACSignal *)signal {
    NSCParameterAssert(signal != nil);
       //创建一个新的信号
    return [[RACSignal createSignal:^(id<RACSubscriber> subscriber) {
        __block BOOL selfCompleted = NO;
        NSMutableArray *selfValues = [NSMutableArray array];

        __block BOOL otherCompleted = NO;
        NSMutableArray *otherValues = [NSMutableArray array];

        void (^sendCompletedIfNecessary)(void) = ^{
            @synchronized (selfValues) {
                BOOL selfEmpty = (selfCompleted && selfValues.count == 0);
                BOOL otherEmpty = (otherCompleted && otherValues.count == 0);
                if (selfEmpty || otherEmpty) [subscriber sendCompleted];
            }
        };

        void (^sendNext)(void) = ^{
            @synchronized (selfValues) {
                                //self和signal都有值才send next
                if (selfValues.count == 0) return;
                if (otherValues.count == 0) return;

                RACTuple *tuple = RACTuplePack(selfValues[0], otherValues[0]);
//发送的值为各自数组的第一个值，这也是和combineLastWith最大的区别
                [selfValues removeObjectAtIndex:0];
                [otherValues removeObjectAtIndex:0];

                [subscriber sendNext:tuple];
                sendCompletedIfNecessary();
            }
        };

        RACDisposable *selfDisposable = [self subscribeNext:^(id x) {
            @synchronized (selfValues) {
//订阅原始信号，如果有值则添加到selfValues
                [selfValues addObject:x ?: RACTupleNil.tupleNil];
                sendNext();
            }
        } error:^(NSError *error) {
            [subscriber sendError:error];
        } completed:^{
            @synchronized (selfValues) {
                selfCompleted = YES;
                sendCompletedIfNecessary();
            }
        }];

        RACDisposable *otherDisposable = [signal subscribeNext:^(id x) {
            @synchronized (selfValues) {
//订阅传进来的信号，如果有值则添加到otherValues
                [otherValues addObject:x ?: RACTupleNil.tupleNil];
                sendNext();
            }
        } error:^(NSError *error) {
            [subscriber sendError:error];
        } completed:^{
            @synchronized (selfValues) {
                otherCompleted = YES;
                sendCompletedIfNecessary();
            }
        }];

        return [RACDisposable disposableWithBlock:^{
            [selfDisposable dispose];
            [otherDisposable dispose];
        }];
    }] setNameWithFormat:@"[%@] -zipWith: %@", self.name, signal];
}

combineLatestWith方法源码：

- (RACSignal *)combineLatestWith:(RACSignal *)signal {
    NSCParameterAssert(signal != nil);

    return [[RACSignal createSignal:^(id<RACSubscriber> subscriber) {
        RACCompoundDisposable *disposable = [RACCompoundDisposable compoundDisposable];

        __block id lastSelfValue = nil;
        __block BOOL selfCompleted = NO;

        __block id lastOtherValue = nil;
        __block BOOL otherCompleted = NO;

        void (^sendNext)(void) = ^{
            @synchronized (disposable) {
//只发送最近的一个值，而不是像zipwith维护一个数组，去数组的第一个值
                if (lastSelfValue == nil || lastOtherValue == nil) return;
                [subscriber sendNext:RACTuplePack(lastSelfValue, lastOtherValue)];
            }
        };

        RACDisposable *selfDisposable = [self subscribeNext:^(id x) {
            @synchronized (disposable) {
                lastSelfValue = x ?: RACTupleNil.tupleNil;
                sendNext();
            }
        } error:^(NSError *error) {
            [subscriber sendError:error];
        } completed:^{
            @synchronized (disposable) {
                selfCompleted = YES;
                if (otherCompleted) [subscriber sendCompleted];
            }
        }];

        [disposable addDisposable:selfDisposable];

        RACDisposable *otherDisposable = [signal subscribeNext:^(id x) {
            @synchronized (disposable) {
                lastOtherValue = x ?: RACTupleNil.tupleNil;
                sendNext();
            }
        } error:^(NSError *error) {
            [subscriber sendError:error];
        } completed:^{
            @synchronized (disposable) {
                otherCompleted = YES;
                if (selfCompleted) [subscriber sendCompleted];
            }
        }];

        [disposable addDisposable:otherDisposable];

        return disposable;
    }] setNameWithFormat:@"[%@] -combineLatestWith: %@", self.name, signal];
}

总结：
逻辑大致相同，有各自应用场景